R – Get NT style domain\user given DN

First, modify your search filter to only look for users and not contacts:

(&(objectCategory=person)(objectClass=user)(sAMAccountName=BTYNDALL))

You can enumerate all of the domains of a forest by connecting to the configuration partition and enumerating all the entries in the partitions container. Sorry I don't have any C# code right now but here is some vbscript code I've used in the past:

Set objRootDSE = GetObject("LDAP://RootDSE")
AdComm.Properties("Sort on") = "name"
AdComm.CommandText = "<LDAP://cn=Partitions," & _
    objRootDSE.Get("ConfigurationNamingContext") & ">;" & _
        "(&(objectcategory=crossRef)(systemFlags=3));" & _
            "name,nCName,dnsRoot;onelevel"
set AdRs = AdComm.Execute

From that you can retrieve the name and dnsRoot of each partition:

AdRs.MoveFirst
With AdRs
  While Not .EOF
    dnsRoot = .Fields("dnsRoot")

    Set objOption = Document.createElement("OPTION")
    objOption.Text = dnsRoot(0)
    objOption.Value = "LDAP://" & dnsRoot(0) & "/" & .Fields("nCName").Value
    Domain.Add(objOption)
    .MoveNext 
  Wend 
End With

[Update April 2016] This answer is now outdated, custom naked domain mapping is supported, see Lawrence Mok's answer.

I have figured it out!

First off: it is impossible to link something like mydomain.com with your appspot app. This is considered a naked domain, which is not supported by Google App Engine (anymore). Strictly speaking, the answer to my question has to be "impossible". Read on...

All you can do is add subdomains pointing to your app, e.g myappid.mydomain.com. The key to get your top level domain linked to your app is to realize that www is a subdomain like any other!

myappid.mydomain.com is treated exactly the same as www.mydomain.com!

Here are the steps:

1. Go to appengine.google.com, open your app
2. Administration > Versions > Add Domain... (your domain has to be linked to your Google Apps account, follow the steps to do that including the domain verification.)
3. Go to www.google.com/a/yourdomain.com
4. Dashboard > your app should be listed here. Click on it.
5. myappid settings page > Web address > Add new URL
6. Simply enter www and click Add
7. Using your domain hosting provider's web interface, add a CNAME for www for your domain and point to ghs.googlehosted.com

Now you have www.mydomain.com linked to your app.

I wished this would have been more obvious in the documentation...Good luck!