First, make sure you mac is running Mac OS 10.6.2. Next, go get the Apple iPhone SDK 3.2 from the apple developer site. This is beta, so expect updates to it. Now, go get MonoDevelop Version 2.2.1 for the Mac. Finally, go get a copy of MonoTouch 1.4.7 (or most recent version). Once everything is installed, go into MonoDevelop and go to Help -> Updates and check for updates on the experimental channel. You should find MonoTouch 1.9 alpha. Download and install it. Now, you should have everything you need. From here, you can create an iPhone application with .NET.
Read more: Wallace B. McClure