Guys, I just got an iPhone a few weeks ago! I'm having a great time with it, and am surprised with the sheer number of quality games out there. Wow.
DrewMG guided me towards GeoDefense, and I've been hooked. It's really challenging and addictive, and I've almost beat all the levels (just one Hard stage left!). Also picked up Pixelogic, a really well-done picross game, and a pseudo-sudoku game called Nine Gap. Oh, and Hero of Sparta, when it was on sale for 99¢, which has been called God of War for the iPhone – cool graphics/presentation, but I just can't come to grips w/ the controls. Oh, and picked up ShapeShape, which is an odd platformer-esque game where you maneuver the little guy by "pushing" him with your touch. The last game I picked up was Zentomino, a tangram game.
Oh, it's not a game, but I picked up an app that gives you access to like hundreds of emoticons – Emoji. Only other iPhone users can see them, though.

GP, I'm with you. I luv my iPhone too.

Insanolord, which app did you decide against?