You must update your Mac Software first
Hi there - I just installed Snow Leopard and Scan Pro ceased working for me too. I read that the workaround is using Preview (which I like much better anyway). I tried that, though, and it didn't work at first.
I found that what I had to do was go to the Apple menu and then click software update. If there were any software updates available, I updated everything. I restarted the computer and then Preview was able to see the scanner and it worked.
So basically, your Mac software must all be up to the minute through Software Update. Like right up to the second. Then Preview will work with your scanner. Worked for me anyhow. Take care.