I think it has more to do with the authentication of the AD. I had similar problems deploying Windows 7 from WDS. Long story short, try to set this policy on the local Windows 7 computers using gpedit.msc

'Computer Configuration\Windows Settings\Security Settings\Local Policies\Security Options\Network security: LAN Manager Authentication Level' change to 'Send LM & NTLM - use NTLMv2 session security if negotiated'

Then reboot, and try the ingroup again.

[edit]
I've tried this myself today at work using a Windows 7 x86 client in our Windows 2000 Domain.

Global groups fail.
Universal groups fail.
Domain Local groups succeed.
[/edit]

Edited by apronk (2009-11-02 09:42 AM)
Edit Reason: Added test results