You can't do it in the Photos app. Here is how to sync and set up photo albums in iTunes that will sync with the iPad. Note that all changes have to happen on the computer.
The Photos app is a bit limited as a photo organizer. Many people choose to use third party apps like Photo-Sort and Photo Manager Pro. Another member recently posted about Photo Shack Pro which looks very interesting, if a bit on the pricey side.
There are going to be some improvement so the Photos app in iOS 5. So if you are the patient sort (or Photos is sufficient once you've got it figured out) you could wait and see what it brings before buying anything..