The MX870 has two components - the printer and the scanner. The printer uses a Canon proprietary protocol called 'canonijnetwork' to advertise the printer on the network. The scanner uses Bonjour. So with the device connected via wireless or Ethernet you should at least see the scanner when you select to Add a printer and have the Default view selected. If the scanner component is not appearing then this is odd, as the printer is visible via the Bonjour view within Safari. If this is so then I suggest you try a reset of the printing system and then try again.
1. Open System Preferences > Print & Fax
2. Move your mouse over the Printers list
3. Hold down the Control button and click the mouse
4. Click the "Reset printing system ..." popup
5. Restart your Mac
Note that this will remove all print queues in Print & Fax but not your installed printer drivers.
Here it is March 2013 and I have had this problem so many times that I bought another printer so there would be at least one reliable machine in my home office.
I've called Canon at least three times to have this resolved and each time, they have helped, but only temporarily. This solution worked immediately! Thanks so much. I've printed it out and will keep it at hand should the problem recur.