You may think it is great to be able to use Mono for iPhone development, but I think it is completely backwards. A non-native toolset will always lag behind (hint: Mono vs. .NET, Moonlight vs. Silverlight). If you can't be bothered to learn Objective-C, a very simple and elegant language; Cocoa Touch, which, as far as frameworks go, is much nicer than .NET; and Interface Builder, which is leaps ahead of Microsoft's way of designing UIs (just IMHO), you are bound to produce crappy bloated apps no one wants to use. Sure, it is interesting--as a proof of concept and a curiousity, but no more than that. Just because you can use C# and Mono to program iPhone, doesn't mean you should.
I disagree. It depends on the application IMO.
Take for example the C9 iPhone app that is being discussed at the minute in another thread. To do that, the person had to go and learn Obj-C, if you could do it in C#, it would look every bit as good as it is only using a few standard controls and wouldn't have required new learning.
Now, I am not saying that if you are serious about iPhone dev that you shouldn't learn Obj-C and Cocoa etc, but for quick apps, things like the tip calculater for example, it seems ideal.