Yep, XP is on its way out, though it works for me for anything I need so I've no reason to upgrade yet - and Samba on Linux is still supported 100%... I've seen those registry tweaks around the place, and most people have had the same results - corrupted files. It seems that the newer SMB implementation is adding extra headers or something.
Maybe someone, some time will get around to fixing SMBFS - the source is on SourceForge. That sort of code is beyond me I'm afraid, but who knows - I might give it a stab some time...