At this point, I could be describing one of many (bad) iPhone games, but Psycho Turnip Games' GravBall [App Store] manages to incorporate some interesting level design that makes the game surprisingly compelling.
Levels start off simply enough but the game quickly introduces a number of additional elements such as death blocks, lasers, switches, moving blocks, and guided rockets.
The game offers 50 progressively difficult levels and is presently a free download.
Even as a free game there are things to criticize: there are no sound effects, the graphics are very simple, and there are even framerate problems. That said, it's the type of game that keeps you trying "just one more time".
The game was originally introduced at $0.99 but quickly dropped to free, so get it while you can.
App Store Link: GravBall, Free