After the NES emulator, you knew it was just a matter of time, and now Doom is running (though not playable yet) on the iPhone.
stepwhite is behind this one, too, and he worked it up because he'd promised a friend he would (not to mention that he was prominent requests for it on Waxy.org, and some other little Apple site you might know). Unfortunately, he hasn't implemented a controller system yet (all you can do right now is watch the demo scenes), but that's the beauty of open source-- no doubt some intrepid programmer out there has an idea on how to do it. Also, you gotta turn that thing sideways, man! This isn't the iPod; let's use the whole screen space.
Geez, after Doom, what's left? Quake III Arena? I stand in awe of what you programmers have been able to do with the iPhone.