There are also scanners like the VuPoint that use a MicroSD card (which you could connect to the iPad via an adapter). I have this one and use it to scan pages from old books at the library.
If you're going to scan sheets of paper, this looks like the better solution. Something like the VuPoint would be better if you don't have something you can feed in as a standard sheet of paper.
Edit: There's a new version with WiFi that's compatible with mobile devices! Cool!