The Mono runtime has been ported to the iPhone OS and a commercial port is available for iPhone developers.
MonoTouch is the edition of Mono for the iPhone that provides access to all of the native iPhone APIs. This is a commercial product and can be obtained from the monotouch web site. There are various editions available:
Additionally, Mono on the iPhone is also available for gaming applications from Unity Technologies
Please visit the http://monotouch.net site for more information on Mono/.NET on the iPhone.