There are hundreds (if not thousands) of Tetris ripoffs, including the iPhone-ready website iphone-tetris.com but there is only one official iPhone Tetris application, and that's the one from Electronic Arts. Let me tell you - it's fantastic.
You can start playing immediately, without even scanning the rules section. Tap a piece, and it rotates. The position of the piece when dropped is ghosted at the bottom of the screen. A downward gesture causes the piece to fall. This is exactly what you want from an iPhone app - a very simple, focused UI which you can master in minutes, not hours.
I can guarantee that the Tetris iPhone app will be a huge timewaster in airports, train stations, long distance backseat drives and medical waiting rooms. Well worth the $7.99 price tag.