After upgrading to Ubuntu 11.10 (64-bit), my Canon Pixma MX870 unfortunately stopped working. Reinstalling the drivers didn’t fix it, nor did a bunch of other things.
However, a fellow by the name of Michael Gruz released drivers for not just the Pixma MX870, but also a truckload of other Canon printers as well (Pixma iP, MG, MP, MX, and Pixus series). Instructions for installation can be found here and the PPA can be found here. These drivers completely solved the issue for me. Problems like these can be extremely frustrating when you’re using Linux, but fortunately there are many dedicated people out there who are willing to donate their time and expertise to fix them.