This weekend I took some time to try and port the MonoDevelop.Debugger.Soft.Unity addins to Xamarin Studio. As it turns out it wouldn’t work, even when the addin compiled and loaded, because it relied on functionality that was added to the base MonoDebugger.Debugger.Soft namespace by Levi Bard; this add-in is built with MonoDevelop/Xamarin so instead I elected to fork the MonoDevelop 4.0 project from github, reintegrate these changes and build a MonoDevelop 4.0 package with the Unity soft debugger included.
Refer to GitHUB if you are interested in the source.
After speaking to on of the kind developers at Unity3D who works on the MonoDevelop port it turns out that they have released some of their plugins for the latest version of Xamarin Studio through their website, as a beta, so that developers can use them.
For this reason I will no longer be maintaining this project. The sources in Github will remain available for users of Unity 3.5 and Unity 4.
Thanks for the support everyone.