"How would you run/debug apps you compiled with VS?" --> Visual Studio could launch or connect to a virtualized iPhone device and let you debug there. That's the approach you use when developing Android apps with Eclipse ... Eclipse connects to a virtual emulated Android device.
I also think if this happens that Visual Studio would support Objective-C (and C/C++) instead of C#/Silverlight on the iPhone. Microsoft would prefer the C# approach but Apple won't allow it.
Virtualization could work, but Apple would have to build in support for it (versus the current approach of running x86 versions of the iPhone apps/frameworks).
I also think if this happens that Visual Studio would support Objective-C (and C/C++) instead of C#/Silverlight on the iPhone. Microsoft would prefer the C# approach but Apple won't allow it.