I just spent a very frustrating couple of hours this afternoon trying to get CUPS to event acknowledge smb printers existed. It would work perfectly on my laptop with Ubuntu 10.04, and but on my Debian Squeeze desktop. In the end I tracked it down to the following:

1. CUPS relies on smbspool. On Debian and Ubuntu this is provided by the smbclient package.

2. The permissions on /usr/lib/cups/backend had been change so it’s contents could only be seen be root (this post put me on to this issue). I’m not sure why this being wrong didn’t break everything. There is also a /usr/lib/cups/backend-available but for some reason there isn’t an entry in this for smb.

Installing smbclient and fixing the permissions on /usr/lib/cups/backend bought everything to life.