Good idea for a thread! I actually don't have an iPhone due to needing a larger screen for work purposes, but I first played this game on my friend's iPhone 4S so I think it qualifies for this thread
Game: Fruit Ninja
I share a house with a few lads in University, and everyone of us has become addicted to this game. So we decided to create a leaderboard on the wall of our living room for each of our highscores (on the arcade game mode, my highscore is 569 but the top score in our house is 989). Yes, we are that bored - but it's a highly addictive game to play!