Usability guru Jakob Nielsen has published his initial findings on the usability of the iPad. He adds the caveat that his findings are based on only limited user testing but I think if you are developing for the iPad it is well worth reading. If you do not have the time his very brief summary captures the situation nicely:
iPad apps are inconsistent and have low feature discoverability, with frequent user errors due to accidental gestures. An overly strong print metaphor and weird interaction styles cause further usability problems.
This fits with my impressions from watching and hearing from iPad users. The first time you try an app there is almost a game to try lots of different gestures (one, two, three finger swipes, pinches, zooms, shaking, tilting) to see if something interesting happens. This is a bit like clicking around on a desktop app to discover a hidden easter egg.
I suppose we should not be too surprised by this since a lot of the current iPad apps were developed by people with little or no actual access to the device. It also helps if you have one or two good examples to follow.