A new version of Mono has been released, version 2.6.3. I assume this will fix the bug with Messagebox.Show(). It's in the list.
According to the Podcast at Delphi.org a new version of Delphi Prism will be released in late April (instead of February as would be the usual rhythm) to match the release of Microsoft Visual Studio 2010 on which it will be based. This new release will most likely come with an updated version of Mono.