You don't need Samba 3 to get PDC, or more accurately windows domain, functionality. This functionality exists in samba 2.2; you just don't get all of the bells and whistles that you'd get with a native windows domain. Samba 3 expands samba's toolset to include more windows domain functions, but by no mean includes all of them. The biggest advantage that Samba 3 offers is integration with windows active directory. However, most people running small to medium sized networks don't need active directory, so there is no real incentive to make the leap to Samba 3 right now.
Other than having the satisfaction of knowing you are sporting the newest version of Samba, I see no pressing need to change to Samba 3. Let it get field test for awhile before making the leap. SME will handle your network just fine in the short term with Samba 2.2x.
Also: Not to be overly academic here, but “SMB” is not Samba. SMB, or Server Message Block, is a network protocol that I believe was created by Microsoft for printer and file sharing. Samba simply uses the smb protocol to talk to other SMB clients..
Regards,
Greg Zartman