For as long as we remember, most new versions of IronPython, IronRuby or the Dynamic Language Runtime exposed new missing functionality in Mono.
After a major release of any of these tools that deeply exercise the internal Reflection.Emit support and the runtime we would implement the missing features and ship a new Mono version.
As part of the Microsoft/Novell collaboration on Silverlight for Linux, a couple of months ago, we received from Microsoft the test suite for the Silverlight class libraries.
We have been working on fixing all of the issues exposed by the test suite. The test suite exposed bugs in Mono and missing features that are necessary for Silverlight in general, as well as the DLR and the DLR-based languages like IronRuby and IronPython.
We are pretty excited about the progress and the increased compatibility of Mono with Microsoft's .NET.