Jetpack Joyride gets boring, but it's fun to play from time to time. I'm positive that there are games out there that are similar to JJR, but i can't think of any off the top of my head.
Offtopic: You should check out GTA 3 for iDevices. It's amazing, has me hooked. Even though i beat GTA on the PS3/PS2/PSP, this is just as fun.