I have recently moved from SBS 2000 to SBS 2003 and had read about and heard of issues with file sharing and non-ms clients. I use MS, Linux and Mac clients to connect to my server. I found a site that is choc full of great articles about SBS and Mac. They have an article related to turning SMB signing off, it is called:
How To Disable SMB Signing in SBS 2003
However for my clients I do not need to turn off signing. I use Thursby's Dave product to connect via Mac OS X 10.3 (Panther) and it supports SMB signing. I will be waiting to upgrade to Tiger until Dave is supported. On Linux, I use mount.cifs instead of mount.smbfs to mount drives and it works just fine. I actual use auto.cifs to automount my windows shares and had to edit the auto.cifs perl script to use cifs and fixup changes in the parameters being passed to the mount command and it now works fine. I did try to get mount -t smbfs (this uses mount.smbfs) and was unable to access the mounted directory, even with signing turned off. It looked like a permission error on Linux box (Fedora Core 3 w/SELinux) since the mount appeared to work, but the permissions on the mounted directory where hookey... Since mount -t cifs works, I just moved forward with that and re-enabled SMB signing on the SBS server for the time being.