Though built on top of Mono, an open-source product, MonoTouch is a commercial endeavor aimed at professional development. Novell seeks to make it attractive through the inclusion of compilers and libraries, which will integrate with the existing iPhone SDK. Developers will be able to make use of both the iPhone API and, through the use of cross-compiling, convert .Net programs into iPhone-compatible software.
This might also open the door for developers who are experienced with .Net and C#, but have not developed on the iPhone. Additionally, it could make it easier to port over existing mobile applications already developed for .Net. As mentioned above, it's a commercial project -- and with a price tag of about $1000 per year, it's not going to be within reach of the solo programmer. Nevertheless, a free development kit may emerge down the road.