I too, have gone through downloading a rented movie to iTunes and then transferring ti to the iPad. When I open the Videos app, it says "No movies." I don't even get the buttons for movies, tv shows, rental.
I tried renting directly to the iPad, but everytime I select a movie from iTunes, the buttons for Purchase and Rent appear disabled. Nothing happens when I tap them.
All I want to do is get a movie on the iPad to watch on an upcoming long flight. I love nearly everything else about the iPad, but thiis just shouldn't be this hard!