One big question that iPhone developers have about the iPad, is whether or not existing iPhone app will also work on the iPad. The answer to that question is YES. The iPad in many ways looks like a large iPhone, and in many ways it acts like one too.
The iPad can run all of Apple's iTunes App Store iPhone and iPod touchs apps. And, just like an iPhone, you can upload your existing apps, music, pictures, videos etc. to your iPad.
When you run your apps in the iPad, you can choose to display them as their original iPhone size (320 x 480) or you can hit a 2x button on the screen and double the size. At 2x the original size, the app will not completely fill the screen.