Dino Run. Here’s an example of a fun, addictive, and pretty simple concept for a game. You’re a dinosaur and you run. There’s also many levels, bonus points for collecting eggs (to save the species), tar pits, big dinosaurs that you can run and jump off of, tunnels, a special key combo to jump extra far, and, oh yes, comets crashing to Earth. But a simple concept otherwise…!
I just heard about the game at the 360|iDev conference. The creator was there demonstrating how to port Flash games to the iPhone and showed off Dino Run for the iPhone (coming soon!)