Biztalk/Webspark are nice initiatives, but not enough.
And I would totally shell out the cash for a MSDN subscription / Visual Studio if I could choose Linux for deployment.
Better yet, collaborate more with the Mono project from Novell. Open-source more of the .NET platform.
The few open-source projects from Microsoft built around .NET don't really have an open-source development methodology ... let others contribute to the DLR, IronPython and IronRuby. Don't fear getting your property tainted ... how many projects from Apache have had such problems, or not getting used by the Fortune "whatever" companies?