iFPS Online was the first online deathmatch game for the iPhone released back in April. The game was a bit rough with less than ideal controls, and limited play areas. It has seen a couple of updates since it was originally released, but is one of the few games that offers online deathmatch play — and as a first person shooter no less. The game has dropped to free for a limited time.
The game allows you to host and join multiplayer deathmatches and also provides a single player mode. If its not enough, the game got its own forum were you can discuss any issues you might have ( with the game of course ), send out some suggestions for the game and learn some tips and tricks. You can find the forum here, and you can download the game here.