After upgrading to 10.6 my Canon Pixma MP620 would show up as a scanner (nice new behavior) but not a printer when trying to connect wirelessly.

After rerunning the old installer (mp620sosxpd7122ea8-2.dmg), I notice a button at the BOTTOM of the Add a Printer window labeled "More Printers...". This is NOT with the other printer types listed across the top. Bad design if you ask me. After clicking "More Printers..." I was given the choice "Canon IJ Network" and my printer showed there along with it address (MAC?).

Maybe this button was showing before I installed the driver, but I don't think so.

Also, before the upgrade it sometimes took more than 30 seconds for my printer to show up in the list of Canon IJ Network printers. If your's doesn't show up right away, wait a bit before giving up.

That button is indeed not present on a default Snow Leopard installation.

Note that the USA and New Zealand websites have new CUPS drivers for the MP620 as well. I see no multifunction hardware listed on the printer page I posted before, so I guess the support site map is the best place to start looking for updated drivers.