I was having problems with my HP PSC 2110 all-in-one printer and in the process of troubleshooting was tinkering with the System Preferences for it. I just randomly clicked on the Scan button and the clicked Open Scanner. Up popped a scan window with 'Scanner is warming up.' Then it showed a preview window and a Scan button. I was completely blown away. I have been trying to get this scanner to work with Leopard and Snow Leopard for years.
This was after applying the most recent HP Printer Drivers v.2.4.1 for Mac OS X v10.6 update, so that is likely the fix.
[crarko adds: I haven't tested this one. It's nice to see the functionality returned to multi-function printers, even if it takes quite a while to happen. Remember to try the latest driver updates when they come out.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100720083750582