I was able to get my MX850 running on 10.6 as network printer.
After the upgrade from 10.5.8 to 10.6 I reinstalled by using my installation CD (which came with the MX850) only the 850 driver and the network tool (custom install). You can download driver and tool via canon-website as well.
After rebooting go to HD/Library/Printers/Canon/BJ Printers/Utilities/Canon IJ Network Tool.
Run Network Tool. Choose “lan”. In the next dialog-window change “Setup” to “Utilities”. After that you will get another dialog box with some weird information. Click “cancel”. Now your MX850 should be detected. Confirm.
After that go to System Prefs/Print-Dialog. Ad a new printer. Wait a second until you see at the bottom of the dialog-window “all available printers”. Click it. Your MX850 will be detected. I confirmed by using “enter” on my keyboard.
It worked for me on three macs running 10.6. My MX850 is hooked via lan-cable to an airport extreme.
Good luck!
Again, see also:
http://discussions.apple.com/thread.jspa?threadID=2131616&tstart=0