As Chris already announced in a previous blog post, SharpDevelop 4.2 and later will support targeting .NET 4.5. One of the interesting new features in .NET 4.5/C# 5 is async. If you have already tried debugging an async method, you will have noticed, that SharpDevelop's debugger is unable to evaluate any local variables.
In short this is because async is - similar to iterator blocks (yield return) - implemented as state machine in a compiler-generated struct. So the debugger has to find the correct location (i.e. compiler-generated code) where all the local variable's values are stored.
In the most recent nightly builds of SharpDevelop 4.2 Beta 2 (starting with build 18.104.22.16861), we've applied the necessary plumbing to make it work. But see for yourself: