The original Wolfenstein is now available for free for the iPhone, officially ported to the device by id's John Carmack. Unfortunately, you need to jailbreak your phone to get in on the fun.
Carmack worked on the port as a pet project, deciding to release it as both free and open source. He's made a few slight tweaks to the game, but it's essentially the same thing you were playing back in 1992.
Next up? A similar port of Doom that he's currently working on. The iPhone might not be best suited for first-person shooters, but how can you say no to free Wolfenstein?