I asked this question on the Microsoft Community and got the following reponse:
I'm afraid you're going to have to talk to the samba folks. There _are_ some
RPC differences with x64, and there's at least one patch out there that has
to be applied in an SBS environment for x64 clients. But obviously MS
doesn't provide patches for samba, so you'll need to post your query in the
samba newsgroup.
I have already posted a question to Novell/SuSE. Will let you know how I get on.