As you may have heard, MFi (Made for iPhone) controllers have started to roll out. In a nutshell, they work with a limited amount of games, but they allow you to use real, physical buttons for titles that may not work as well on the touchscreen. Think games like Bastion or Limbo, which would benefit greatly from controller support, and you have a general idea of what these devices are trying to accomplish.
Right now there are a few controllers in the works, but the major two players that are out currently are the Logitech Powershell, and the MOGA. I spent the past week testing out both extensively, and in the end, I have one clear-cut favorite.