Currently Mono offers two GUI toolkits, Windows.Forms and Gtk+. Both of these work on OS X, but they only run via an X server. Just like early Java GUI toolkits, the non-native look and feel is causing problems for developers.
Miguel de Icaza has announced that Mono 1.2.6 will be shipped with an OS X native backend for the Windows.Forms implementation. They are also working with Imendio's project, Gtk+ for Mac OS X. The latter is currently a point where MonoDevelop can run entirely without X11 on the Macintosh.
A port of MonoDevelop is also planned for Windows, specifically for developers working on Gtk+ projects. Miguel recommends Windows developers continue to use Visual Studio or SharpDevelop for non-Gtk+ projects.
Lower priority project for the Mono team include CocoaSharp, which provides native bindings for the Cocoa API, and Objc-Sharp, a bridge between Mono and Objective-C.