I'am very confused. Are you not able to test like you have to or aren't you interested in business customers? Since DSM 4.0 it is not able to join a Windows 2008 Domain. As i remeber, we had the same problem from 2.3 to 3.0 or 3.0 to 3.1 (not sure anymore).
What I did:
Two new 1512+. NAS1: 192.168.7.41 / NAS2: 192.168.7.42, Gateway: 192.168.7.1, DNS: 192.168.3.2 (DC although 192.168.3.2).
Install NAS1 with 4.0, try to join the domain, ERROR.
Install NAS2 with 3.2, try to join the domain, OK. Look if I am able to see the users & groups from domain, OK. Update to 4.0 - still OK.
So, why i am not able to join the domain with NAS1? Why I am not allowed to downgrade? (because then I can do same like with NAS2)
For luck I was able to test this on non productive NAS. What to do if I am not able to test this - we don't have unused NAS anytime - and upgrade the firmware from a productive NAS and get problems like this? No one would be able to access the NAS and I am not able to downgrade to go back to the stable version.