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.
https://bugzilla.novell.com/show_bug.cgi?id=566247 Once it is marked as resolved, you know your request can be satisfied too.