That doesn't work out of the box. I think it has to do with the password hash formats not being the same. CIFS doesn't send passwords through to the server, just hashes, so can't take the password and hash at the server either for comparison.
Note: I haven't tried it. Instead we set up a Samba server (and passthru to it) and users just have to remember which is their samba password and which is their alfresco password.
