As documented by Canon, if you upgrade from 10.6 or 10.7 to 10.8, you need to remove the older Canon UFR2 driver and then install the latest version that supports 10.8.
First you will need to open Print & Scan and remove the current iR2520 print queue.
Then open Finder and navigate to HD > Library > Printers > Canon and trash the UFR2 folder.
Sill in Finder, navigate to HD > Library > LaunchAgents and trash the file "jp.co.canon.UFR2.BG.plist".
Then restart your Mac and install the UFR2 v2.43 driver. With the driver, create the print queue again, select IP > LPD for the protocol. This new version should allow you to print from 10.8.