Domain Admin login failure.

I installed SQL Server 2005 RTM on Windows Server 2008. I immediately
rebooted the server and installed SQL 2005 Service Pack 2; I did not run the
Vista wizard at the end.

I am having an issue where I can log into the SQL server using the SA
password or the local administrator account, but get a login failure when
trying to logon using a domain admin account (domain admins is in the local
administrators group).

Any ideas?