To enable C# development on the Mac, I have used the MonoTouch tools from Xamarin. The current user interface certainly does not meet Apple design guidelines, but this is what the simple unit converter application looks like in the iPhone simulator:
This being my first attempt at C# development on the Mac, I have experienced quite a number of pitfalls. At this point I am not sure if these problems are due to limitations and issues in the MonoTouch components, or whether the problems are due to my limited experience with the platform.
Nevertheless, the unit converter works as intended, providing immediate unit conversion for all quantities and units currently represented in the csunits library. The source code, including a MonoDevelop solution (csunits-monotouch.sln) and MonoDevelop projects, are available in the Github repository.
Actual deployment on a real iPhone or iPad has not yet been tested. Phone deployment is not for free. For this proof-of-concept, I considered running on the iOS Simulator to be enough.