I recently upgraded to MonoDevelop 2.2 Beta 2 (for the debugging capability, more on that later) on my Mac.
When I opened a Moonlight solution and tried to run it, I got the following error:
Could not find respack
The project had lost its references to System.Windows and System.Windows.Browser.
The cause turned out to be that when you upgrade MonoDevelop, it doesn't always update where it's pointing.
The solution was to go to:
/Library/Frameworks/Mono.framework/Versions
and delete all directories except the one for the current version of MonoDevelop, in my case, 2.4.2.3