Just spent an hour trying to figure out why my iOS project would not build in Xamarin Studio for OSX. I had just worked on Android and went back to iOS. I cleaned and removed my obj / bin over and over but it did nothing. I tried to re-create one of the XAML views and I got an error that it was already in the project in the Android obj folder. This clued me into the conflict so I "unloaded" droid and iOS built with no code changes.
Something needs to be fixed on Xamarin's end in regards to this.
Just FYI, when I was getting the build error it would say "object reference not set to an instance of an object" for each view / xaml.cs combo.