I like the idea of using the iPad as a command interface – it suits an element of personal roleplaying – and particularly suits any "war game" table top simulation. I think the 2d physics games that are popular as of late will adapt well – e.g. world of goo, crayon physics – the idea of a play area controlled by touching. Hidden object (zoom and pan) and match 3 games / block swap games could do well too.
I think theres more scope for functional (less games orientated) apps than the iphone, the big interface suits it better.