I have a CANON PIXMA MX-850 network connected. Was working fine, on 10.5, upgrade to 10.6 deleted my printers. After that the Canon showed up as a scanner only. Same behavior everyone else is seeing. Very annoying.

Go directly to canon and load their latest and greatest drivers. The released a bunch of new software in Sept of 2009. This still didn't resolve the issue. On a lark I downloaded the IJ Network took again. That is the tool that lets you set your printers network settings remotely. It installed, forced a reboot ran the software and I just confirmed the settings that it found. After that my printer showed up in the default list after that. Seems to work fine now.
Rod

Just in case someone made the same mistake I did:

After Canon posted their new driver for the all-in-one mp620 printer/scanner I ASSUMED that the one driver worked for the printer/scanner. After installing the driver at the top of the list I could print again - yay! But, I still couldn't scan! After a few days of grumbling about still not being able to scan I went back to the Canon website and read a little more carefully - guess what? This all-in-one device requires two, separate drivers! I downloaded the SEPARATE driver for the scanner component and it worked! After 1 month of not being able to print and/or scan - I'm finally back up and running!