XBMC is now available as a jailbreak app for the iPad and iPhone 4. XBMC an open source media center application that runs on almost every OS. It is on Linux, OSX, Windows.
The iPad version also offers full hardware accelerated video playback and network access to media. The iPhone 4 is not ready for primetime yet. It seems a tad bit buggy to me.
The app is for jailbroken devices only. It will not be in the App Store, you have to go to Cydia.