Why are we telling you this? For many, the idea of having a competent reproduction of Settlers of Catan on their iPhone would be a dream come true. For a while there was a competent facsimile in the shape of Kolonists (which has seemingly been removed from the App store) but now, thanks to USM and Exozet games there is an official title available dubbed (iTunes link). The official game is $4.99, features the original rule set from the board game, supports "hot seat" multiplayer (which means you pass the iPhone around, there's no Wi-Fi or Bluetooth support yet) and, for newbies, there's an extensive and very clear tutorial mode that will teach you how to play.
Reception from fans so far has been very positive.