Just came across this very comprehensive guide for using C# and Mono to develop for the iPhone, iPod Touch and iPad.
This is definitely something I am very interested in getting in to when I have some spare time. One of the questions I have though is if I have to really buy a Mac and use this for my development. I work on a fantastic workstation at work equipped with quad displays. I'm afraid moving to a Mac to do my development just isn't appealing to me in the slightest.