As the Samba Wiki notes, Windows 7 clients are supported as members of a NT4-style domain since Samba 3.3.5 and Samba 3.4.0 if the following registry changes are made:
[HKEY_LOCAL_MACHINESystemCurrentControlSetServicesLanManWorkstationParameters] "DNSNameResolutionRequired"=dword:00000000 "DomainCompatibilityMode"=dword:000000001