Can we install the new XCode supporting iPhone 5 and have it work with MonoTouch 5.4 to create apps that will work on the new screensize?
Redth# has posted a nice writeup on getting your MonoTouch app ready for iPhone5. The key change to your project is to add a new splash image for the new display size, Default-568h@2x.png. I'm sure Xamarin will release a new release that adds a GUI to set this, but you can manually do it now following Redth#'s instructions.