Remember DOOM? You might not if you're a longtime Mac user, since the attempts to bring it to the Mac really only met the dictionary definition of "port." Yeah, you could play it, but it wasn't any fun.
Then Bungie came along and decided to take their own shot at it. I'm sure you remember that one. It was called Marathon. You may also remember what happened to Bungie: Microsoft wrote a check and that was that. But right before that happened, Bungie open-sourced Marathon, which runs on Mac OS as Aleph One. So you could still get it on your Mac, but only on your Mac.
TouchArcade is reporting one of their forum threads contains the news that developer Daniel Blezek is actually working on bringing this to the iPad. He has also posted that it hasn't been a completely smooth process, and there's still a question of GPL licensed code being sold on the App Store. But if you have an iPad and a whole lot of spare time, as of yesterday afternoon Daniel said he was still looking for beta testers.