Turn-based rpgs usually work pretty well since theres not much action/movement needed.
There are games that use a small screen to their advantage (create a challenge by displaying limited screen) I think Tiny Wings does a great job of this. You need quick reactions to make adjustments and what not.
I really suggest developing/designing an app for iphone and then attempting to port it to ipad.
I take it a step further and design all my apps for iphone 3.5 inch screen and then scale up otherwise its just too hard to move down.