To accompany the in-depth walk-through on MonoTouch development for iPhone devices using C# posted earlier, I’ve also uploaded a much shorter introductory article on the same subject.
It could be a good starting point if you want to get started quickly.
The article can be found here.
Update: This article has been out of date for some while, thanks to MonoTouch having moved from v3 to v5 and Xcode having moved from v3.x to v4.x and markedly changing. I've now updated this introductory article and the updated version can be found here.