iPhone app using .NET development environment?

If you just want to leverage some knowledge of c#, you can use Unity. Games developed with Unity (http://unity3d.com) use Mono with an AOT (ahead-of-time compiler) to compile mono bytecodes to ARM. (The apps created by Unity have a shell that is, in fact, a fairly conventional cocoa touch app created in Objective-C++ and XCode.) It's not a dot net app, but you can use your existing knowledge of c#.