Ouch, that really got me - I wanted to play with the GUI tools, but MonoMac doesn't fully support XCode 4 yet and only has this to say:
Developers that use Interface Builder are recommended to install Xcode 3.2.6 for the time being. MonoTouch News.
The bad part: that's from March. That's way too long without support, so MonoMac is just half a tool (if at all). Running XCode 3 and XCode 4 in parallel is also not an option (both want to be in the same folder). De facto, you're either stuck with XCode 3, or MonoMac is out. Or you build your GUI with GTK# - then XCode doesn't matter (the compiler part of XCode 4 is supported), but then you can forget about the AppStore (and GTK# doesn't really look great). Damn.