I am using openSUSE 12.1 and MonoDevelop (2.* and 3.*) displays an exception dialog at startup saying /MonoDevelop/Core/PlatformService experienced a problem. In fact, if you launch MonoDevelop from the Terminal (execute monodevelop at prompt), more information will be displayed.
So the conclusion for me is simple, that MonoDevelop failed to find libgnomeui.*.so on this machine, which can be easily fixed by installing this package libgnomeui,
1 zypper install libgnomeui
You may hit another problem, but the message got from the Terminal should be your guide.