I'm hoping to get mine soon, but when I played with the iPad in the Apple Store a couple of weeks ago, I realized that the placement of the keys is a little different than on the iPhone. (I don't remember exactly which buttons messed me up, but I think having the numbers on the top row, which makes sense with the large screen, was the problem.) That really threw me, and I know it will take some getting used to. Also, I think the Delete button being in a different position was difficult.
I think the iPad's arrangement makes more sense than the iPhone's, but after using the iPhone for almost 2 years, I guess I've just gotten used to it.