UPDATE:

Some people may have been successful with connecting to a samba PDC with Samba. Check out the following URL:

http://www.nabble.com/Windows-7-RC-td23405949.html

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I've been trying for the past week or so to get Windows 7 to connect to our samba domain controller (pdc) without much luck.

The error is "The following error occurred attempting to join the domain "DOMAINNAME": The specified domain either does not exist or could not be contacted".

Browsing shares via \\server\sharename work without a problem, and the Vista fix of changing the 'Network Security: LAN Manager authentication level' policy to 'LM and NTLM - use NTLMv2 session security if negotiated' hasn't helped at all.

I finally got annoyed enough to email the samba support lists (as my google-fu wasn't helping much) and found the answer which was in the samba-technical list: http://lists.samba.org/archive/samba-technical/2009-January/062827.html

Sincere apologies, but wanted to confirm that there is an issue with NT Domain support in
Windows 7. We're trying to expeditiously track down exactly the issue(s), but the short of
it is I wouldn't spend time trying to get this functionality to work. We'll get some sort
of official notice out shortly (and I do mean shortly).

Jim Pinkerton,
Microsoft

So, I guess we wait and hope it's fixed before release.. or my company isn't touching Windows 7 (much like Vista).