Thanks to our friends at Novell and the Mono project:
Available immediately, monoTouch enables applications developed in any .net language to run on the iPhone. Significantly, monoTouch provides .net bindings to native API, allowing application developers will have access to iPhone specific functionality from within their .net applications. monoTouch integrates with both the free MonoDevelop IDE as well as Apple's XCode toolkit.
Applications developed using monoTouch compile completely to native code – they are not JIT compiled or interpreted.
This is a commercial offering, not an open-source appliction, and will set you back $399. But with over a million iPhones, and a few more million iPod Touches, a good app can recoup that pretty easily. This is for Mac OSX only, not Windows.
Find it at http://monotouch.net/.