Even our beloved Snow Leopard had issues:
An Active Directory user may not be able to log in to Mac OS X v10.6.0 or Mac OS X v10.6.1 client. This can happen when the Active Directory connector in Directory Utility is configured to “Create mobile account at login,” and a Home folder is specified in Active Directory for the user.
The fix? Waiting around for Mac OS X 10.6.2.