The UI design is very different - read the development guidelines. For one thing, tab bars are generally out, replaced by a tab bar at the top. Also, you are MUCH more strongly encouraged to develop for all orientations.
Basically though, the flow of things is very different because instead of a large drill-down tree as on the iPhone, you have a lot more space to lay out items in front of you. Pretty much the whole UI should be thought out although it may make sense to re-use some table view code in presenting side lists for something like a split view.
Also consider that people on the iPad may be much less casual users of your application, on the phone people tend to be in and out of applications pretty quickly but on the ipad a user is usually more settled in to do something long term. So you may want to think how to optimize the iPad for users who are doing something more complex or in the app a longer time.