Not that this would actually happen. But it sure is fun to think about.
Certainly with all the efforts to port llvm to windows, it would be possible to get the needed objective-c compiling working under studio.
Visual Studio already knows how to cross compile for different platforms, so the "infrastructure" is already in place for VS2010 to produce non-windows, non-intel binaries.
None of it is un-doable, and it would open up iPhone development to a large number of people that just don't want to learn XCode (which is the actual stumbling block that I hear about most often. XCode is just too foreign when compared with VS).
Oh well, that was fun. Back to reality.
... and by the way, isn't it strange that no matter what kind of picture is taken of Ballmer, he always looks kind of goofy?
Personally I'm against a flood of .NET mercenaries (especially VB) into the App Store.
It's unlikely that Apple would allow .Net code to be compiled down into Objective-C, or run natively on the iPhone. If this happened it would be more likely that Apple would allow it's frameworks and the Objective-C compiler to be accessible to Visual Studio (you would target the iPhone in your project/solution).