As stated in this question, thanks to some compilation trickery, Visual Studio 2010 allows a project targeting at .Net 3.5 to benefit from optional parameters.
Is it possible to achieve similar behavior with MonoDevelop 2.6b1?
EDIT:
This problem doesn't however affect xbuild
which can successfully leverage dmcs
(C#4.0 compiler) to target a 3.5 profile.