Today's MonoTouch release will let you install Xcode 4 and continue building applications with MonoTouch.
But if you are using the Xcode 4 installation, you will not be able to edit your XIB files. We advise developers to install the new version of Xcode 3, which contains support for iOS 4.3 and still comes with Interface Builder.
We are working to create a new integration system from MonoDevelop to XCode4 (since they have now integrated Interface Builder with Xcode)