PAHU has the answer.

Okay, have done some testing and found some interesting things.

With a new install of 10.6.5 (selecting not to add any printers) I was not able to locate any Canon AIO via wireless. The Default Add Printer pane was empty so it looks like the printer and scanner driver needs to be present on the Mac. For the printer this is understood, as the Canon protocol ‘canonijnetwork’ is not present in the BJPrinters folder located in Library > Printers > Canon (due to the customized installation the Canon folder was not even present). In checking the Image Capture folder (HD > Library) I also found that the Devices folder was missing. Inside of here you would normally find the Canon IJScanner package/s for a standard installation of 10.6. But with the customized install this package was not present and appears to influence the Mac’s ability to ‘see’ the networked scanner component. To confirm this, I installed the Canon v2.4.1 driver update, which in turn created the Devices folder in Image Capture and consequently made the scanner component appear in the Default Add Printer view.

Another observation was that prior to installing the Canon v2.4.1 update, Image Capture would still display the MP on the network, but under the Shared menu – not the Devices menu. So even without the ICA driver installed nor the MP scanner driver installed, I was still able to use Image Capture to scan a document from the wireless MP. The same was true for Preview in as far as the Networked Scanner selection showed all the MP’s present on the wireless network.

At this stage I believe that the Canon driver update you mentioned may have contributed to part, if not all, of your current symptom. The install.log does have a record of what this update installs. If you checked it there may be record of the installer not completing correctly. In any case I assume that for your Mac this update did not complete correctly and is causing the problem. So one suggestion would be to run the Canon Driver v2.4.1 update again (or for the first time if you simply used Software Update previously) to see if this repairs the CanonIJScanner packages.

via Apple – Support – Discussions – Canon MP560 no longer recognized by ….