There is an attatchment called the iControlPad for the iPhone.
All Emulated games will play easier and better with it. I also thinks it opens up the option of adding two or maybe three more emulators to the iPhone.
Does the iPhone have the power to emulate:
Nintendo DS
Playstation Portable
Wonderswan Color
If it has just the POWER to emulate one of these then when iControlPad is released, playing the games should be very uncomplicated.