Ldap – Can Active Directory issue a LDAP redirect to another LDAP server

This is a good place to start: http://technet.microsoft.com/en-us/library/cc756101(WS.10).aspx.

This tool is quite outdated (it was made for Windows 2000 and only knows about < 1 Ghz CPUs), yet still says that 3-4 DCs will be more than enough for 500,000 users of which 50,000 are active daily. I don't think you should have any problem managing such a database with today's hardware.

the technology you are looking for is AD LDS (in 2008 speak) or ADAM (in 2003 speak) There are differences but you can only deploy AD LDS on a 2k8 system. ADAM can be installed on XP and 2003 (although I haven't tried it on windows 7 or vista I suspect it will work)