I've just been battling with windows auth on samba shares from a regular lion (not server) box, and what cracked it for me go network->advanced then the WINS tab and enter the desired netbios name and workgroup name in there. This is after I already tried a macports install of darwin3 with no joy.
Hope that helps
Populating the NetBios and workgroup names appears to make a difference when a Windows machine fails to log into the Mac over SMB.
I should note that some non-Windows devices that connect to the Mac over SMB may not trigger this problem: I've successfully used ES File Explorer from several Android devices to connect to the same Mac that I couldn't connect to from a Windows machine until I populated the NetBios and workgroup names.