現時点ではまだサポートされてないようですね。

http://ios.xamarin.com/Roadmap

Short Term

Some of the technology that was developed for Xamarin’s XTouch and Xdroid products will be folded into MonoTouch and Mono for Android, and we will deliver these changes initially on our beta channel, and we will graduate those to the stable channel over time.

For MonoTouch, this will include the following improvements:

  • New MonoDevelop Release:
    • Adds integration XCode 4’s Interface Designer
    • Adds new iPhone property list editors
    • Unified MonoDevelop, Mono and MonoTouch updater

全く見た目が変わっててさっぱり使い方がわからなかった、、、。
InterfaceBuilderってどこいっちゃたの? って感じです。