Honestly I think it depends on the discipline of the developers.
I use Subversion for my personal work and I've used it at a few jobs. What I like about Subversion is I don't have to hunt someone down and ask them why they're working on something and if it would be OK for me to do some work. The problem comes when someone decides to start working on something and doesn't check it in for a while; this can make merging difficult as several changes get made between their check-out and check-in.
I use Perforce right now and for some reason I like SVN better. Perforce definitely gives me a better indication that there's going to be merge conflicts, and even has built-in tools to help me resolve the merges. It has the same problem where if someone makes tons of changes over a long time, the merge will be more difficult.
Basically both models require you to check in changes often. If you make numerous check-ins, then you reduce the likelihood that you'll require a merge. I'm guilty of keeping stuff checked out for too long way too often. Personally I feel like SVN's price tag makes up for anything it lacks compared to Perforce; I haven't found a difference between them yet.
I was previously using the top-voted answer, but it needs a bit of cleanup, so here it is redone for Xcode 4, with some improvements.
I've researched every file in this list, but several of them do not exist in Apple's official Xcode documentation, so I had to go on Apple mailing lists.
Apple continues to add undocumented files, potentially corrupting our live projects. This IMHO is unacceptable, and I've now started logging bugs against it each time they do so. I know they don't care, but maybe it'll shame one of them into treating developers more fairly.
If you need to customize, here's a gist you can fork: https://gist.github.com/3786883
#########################
# .gitignore file for Xcode4 and Xcode5 Source projects
#
# Apple bugs, waiting for Apple to fix/respond:
#
# 15564624 - what does the xccheckout file in Xcode5 do? Where's the documentation?
#
# Version 2.6
# For latest version, see: http://stackoverflow.com/questions/49478/git-ignore-file-for-xcode-projects
#
# 2015 updates:
# - Fixed typo in "xccheckout" line - thanks to @lyck for pointing it out!
# - Fixed the .idea optional ignore. Thanks to @hashier for pointing this out
# - Finally added "xccheckout" to the ignore. Apple still refuses to answer support requests about this, but in practice it seems you should ignore it.
# - minor tweaks from Jona and Coeur (slightly more precise xc* filtering/names)
# 2014 updates:
# - appended non-standard items DISABLED by default (uncomment if you use those tools)
# - removed the edit that an SO.com moderator made without bothering to ask me
# - researched CocoaPods .lock more carefully, thanks to Gokhan Celiker
# 2013 updates:
# - fixed the broken "save personal Schemes"
# - added line-by-line explanations for EVERYTHING (some were missing)
#
# NB: if you are storing "built" products, this WILL NOT WORK,
# and you should use a different .gitignore (or none at all)
# This file is for SOURCE projects, where there are many extra
# files that we want to exclude
#
#########################
#####
# OS X temporary files that should never be committed
#
# c.f. http://www.westwind.com/reference/os-x/invisibles.html
.DS_Store
# c.f. http://www.westwind.com/reference/os-x/invisibles.html
.Trashes
# c.f. http://www.westwind.com/reference/os-x/invisibles.html
*.swp
#
# *.lock - this is used and abused by many editors for many different things.
# For the main ones I use (e.g. Eclipse), it should be excluded
# from source-control, but YMMV.
# (lock files are usually local-only file-synchronization on the local FS that should NOT go in git)
# c.f. the "OPTIONAL" section at bottom though, for tool-specific variations!
#
# In particular, if you're using CocoaPods, you'll want to comment-out this line:
*.lock
#
# profile - REMOVED temporarily (on double-checking, I can't find it in OS X docs?)
#profile
####
# Xcode temporary files that should never be committed
#
# NB: NIB/XIB files still exist even on Storyboard projects, so we want this...
*~.nib
####
# Xcode build files -
#
# NB: slash on the end, so we only remove the FOLDER, not any files that were badly named "DerivedData"
DerivedData/
# NB: slash on the end, so we only remove the FOLDER, not any files that were badly named "build"
build/
#####
# Xcode private settings (window sizes, bookmarks, breakpoints, custom executables, smart groups)
#
# This is complicated:
#
# SOMETIMES you need to put this file in version control.
# Apple designed it poorly - if you use "custom executables", they are
# saved in this file.
# 99% of projects do NOT use those, so they do NOT want to version control this file.
# ..but if you're in the 1%, comment out the line "*.pbxuser"
# .pbxuser: http://lists.apple.com/archives/xcode-users/2004/Jan/msg00193.html
*.pbxuser
# .mode1v3: http://lists.apple.com/archives/xcode-users/2007/Oct/msg00465.html
*.mode1v3
# .mode2v3: http://lists.apple.com/archives/xcode-users/2007/Oct/msg00465.html
*.mode2v3
# .perspectivev3: http://stackoverflow.com/questions/5223297/xcode-projects-what-is-a-perspectivev3-file
*.perspectivev3
# NB: also, whitelist the default ones, some projects need to use these
!default.pbxuser
!default.mode1v3
!default.mode2v3
!default.perspectivev3
####
# Xcode 4 - semi-personal settings
#
# Apple Shared data that Apple put in the wrong folder
# c.f. http://stackoverflow.com/a/19260712/153422
# FROM ANSWER: Apple says "don't ignore it"
# FROM COMMENTS: Apple is wrong; Apple code is too buggy to trust; there are no known negative side-effects to ignoring Apple's unofficial advice and instead doing the thing that actively fixes bugs in Xcode
# Up to you, but ... current advice: ignore it.
*.xccheckout
#
#
# OPTION 1: ---------------------------------
# throw away ALL personal settings (including custom schemes!
# - unless they are "shared")
# As per build/ and DerivedData/, this ought to have a trailing slash
#
# NB: this is exclusive with OPTION 2 below
xcuserdata/
# OPTION 2: ---------------------------------
# get rid of ALL personal settings, but KEEP SOME OF THEM
# - NB: you must manually uncomment the bits you want to keep
#
# NB: this *requires* git v1.8.2 or above; you may need to upgrade to latest OS X,
# or manually install git over the top of the OS X version
# NB: this is exclusive with OPTION 1 above
#
#xcuserdata/**/*
# (requires option 2 above): Personal Schemes
#
#!xcuserdata/**/xcschemes/*
####
# Xcode 4 workspaces - more detailed
#
# Workspaces are important! They are a core feature of Xcode - don't exclude them :)
#
# Workspace layout is quite spammy. For reference:
#
# /(root)/
# /(project-name).xcodeproj/
# project.pbxproj
# /project.xcworkspace/
# contents.xcworkspacedata
# /xcuserdata/
# /(your name)/xcuserdatad/
# UserInterfaceState.xcuserstate
# /xcshareddata/
# /xcschemes/
# (shared scheme name).xcscheme
# /xcuserdata/
# /(your name)/xcuserdatad/
# (private scheme).xcscheme
# xcschememanagement.plist
#
#
####
# Xcode 4 - Deprecated classes
#
# Allegedly, if you manually "deprecate" your classes, they get moved here.
#
# We're using source-control, so this is a "feature" that we do not want!
*.moved-aside
####
# OPTIONAL: Some well-known tools that people use side-by-side with Xcode / iOS development
#
# NB: I'd rather not include these here, but gitignore's design is weak and doesn't allow
# modular gitignore: you have to put EVERYTHING in one file.
#
# COCOAPODS:
#
# c.f. http://guides.cocoapods.org/using/using-cocoapods.html#what-is-a-podfilelock
# c.f. http://guides.cocoapods.org/using/using-cocoapods.html#should-i-ignore-the-pods-directory-in-source-control
#
#!Podfile.lock
#
# RUBY:
#
# c.f. http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/
#
#!Gemfile.lock
#
# IDEA:
#
# c.f. https://www.jetbrains.com/objc/help/managing-projects-under-version-control.html?search=workspace.xml
#
#.idea/workspace.xml
#
# TEXTMATE:
#
# -- UNVERIFIED: c.f. http://stackoverflow.com/a/50283/153422
#
#tm_build_errors
####
# UNKNOWN: recommended by others, but I can't discover what these files are
#
Best Answer
Natch - the version-control and any other infrastructure code is also under version-control but I would use a separate project from any development project.
I prefer a searchable wiki or similar knowledge-base repository to clogging up your bug-tracking system with things like VCS config.
Most importantly, make sure that the documentation is kept up to date - in my experience, people are vastly better at keeping code docs up to date than admin docs. This may have been the individuals concerned . One thing that is often overlooked is, if systems are configured according to standard Unix Practices or similar philosophy, that implies a body of knowledge about locations that may not be familiar to an OS/X or Windows programmer, faced with suddenly fixing a broken script. Without being condescending, make sure basic assumptions about location and interdependency are documented.