In case anyone else is having problems, I had trouble with getting Mission Planner to work on Linux after about version 1.2.36. I was getting various "System.TypeLoadException" errors.
Anyway, as I understand, MP has been upgraded to use a newer version of .net and that means some other packages need installing.
So, to ensure MP 1.2.36 and up works properly on Ubuntu (tested on my Ubuntu Quantal laptop and Ubuntu Precise netbook) install the following packages:
sudo apt-get install mono-runtime libmono-winforms2.0-cil libmono-corlib2.0-cil libmono-system-web4.0-cil libmono-system-management4.0-cil