The challenge of .NET

Sort of open, sort of not Microsoft's certainly taken up XML as a key ingredient in their long-term strategy. They seem to feel that they can build a largely-closed system on an open infrastructure and an open data format.
Some parts accessible The message formats for .NET seem (so far) to be open and accessible, though the contents of those messages may be obscured. At this point it seems possible to build competing infrastructures which interoperate with .NET.
Some parts need challenge The "Hailstorm" vision builds on much of the same infrastructure, but gives Microsoft the keys to personal information. This seems like an arena where open source challenges are viable, especially given the amount of effort in the peer-to-peer and messaging fields.