It looks like a "big iPhone," and apparently it acts like one too: the iPad
can run traditional iPhone apps completely unmodified, and can even zoom them up to full screen. Additionally, a new iPhone SDK is out today to allow developers to tweak their apps for the specifics of the iPad. Of course, Apple has rebuilt its apps from the ground up for the iPad, and developers can do the same with the SDK.
Check out our hands-on with the iPad here
New iPhone & iPad SDK