At some point, my Mac developed a problem: I couldn't connect to Neil's PC over the home network. No file sharing or printing. Googling suggested that this was a Samba issue, known and unfixed.
When I upgraded to Leopard, i had the same problem. Until yesterday, when I noticed that Neil's PC was showing up in the sidebar. I tried adding his printer again, and it all works! I'm happy.
I have no idea why it stopped working (it worked perfectly for ages, then I started having problems--either I couldn't see his PC at all or if I could, I'd get SMB errors) nor why it started working again, but I'm glad it is!