I have the same issue with my HP AIO C6380. After a lot of reading on the HP site, turns out my scanner function, as well as many others, aren't supported in Snow Leopard. The only solution I've found, without having to buy or install another program, is to use the Image Capture program in the Applications Folder. It worked for me when nothing else would. I found it here:
http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=c01910186
Here is the actual info:
Method two: Scan from the Image Capture application
1.
Click the Apple Image Capture icon in the dock, or open Image Capture from the Applications folder.
2.
Select your product from the list displayed. If you are using a networked device, select it from the Shared list.
The scan interface opens.
NOTE: You can click Show Details to view settings and options.
Although I like HP, I might have to reconsider the brand of my next AIO.