Then on Monday I finally got round to upgrading our iMac G5 from 10.4 to 10.5. (That took about six hours, incidentally, including the 10.5.8 update which I already had on a thumb drive and a bunch of others that came via Software Update). After installing the latest Canon drivers and related software, guess what? It wouldn't scan, and the symptoms were exactly the same as on my 24in iMac.
Much head-scratching followed, and a few choice words were uttered.
After trying a few things such as checking the firewall settings, I looked in the logs and found the user diagnostic reports included references to an Epson-related file in /Library/Image Capture/TWAIN Data Sources. I don't have an Epson, but it's quite possible that somewhere along the way I did use an Epson all-in-one with that machines while reviewing the device. And I'd certainly installed Epson and Brother software on the 24in iMac for the same reason, and both were mentioned in the logs.
Opening /Library/Image Capture/TWAIN Data Sources and deleting all the files relating to scanners other than the MP800R did the trick on both iMacs, restoring normal operations.
So what had happened? Please read on.
It would probably have been better to move the files to another location (eg, in a folder in ~/Documents) in case they're ever needed. But I'd expect that (re-)installing the other devices' software would put everything back in the right place anyway, so I doubt any harm has been done.
I also suspect that the problem isn't specific to Canon's software. If you find yourself unable to scan or to set the TWAIN device to the scanner or all-in-one that you're actually using, try cleaning up the /Library/Image Capture/TWAIN Data Sources folder.
The real moral of the story is that if you're going to manually uninstall scanner drivers from a Mac, make sure you do the job thoroughly or it may turn around and bite you sometime down the track.