The Canon PIXMA MX850 (and other printers from Canon it seems) have an ethernet port, but do not support any standard network printing protocol, therefore they are not supported in cups out of the box. The protocol used is called BJNP, and uses tcp/udp ports 8611 to 8614.
Fortunately, an open source BJNP driver for cups has been written. You'll find it here, along with install instructions:
http://www.fazant.net/cups/.
Once you have BJNP support in Cups, your printer should be auto-detected. You'll be able to print using the Gutenprint PIXMA iP5300 driver.
The following sources provided useful information to me:
-
OpenPrinting MX850 page
-
OpenPrinting Forum post
-
Ubuntu forums thread
-
BJNP protocol backend for CUPS README
