Rich,
I run Act locally on 6 PC's and the Act database is sitting in an ibay on SME V5 server. You should have no problems as long as you are running the more recent versions of SME with Samba 2.2.1a or higher. The file sharing stuff is really a Samba issue, so the recent versions of Samba handle file locking and other such issues with no problems. Let me know if you have any problems.
Patrick