It's a good move for the segment. While we aren't particularly in the market for HPC-type stuff, it's a very nascent market. One where Linux et al are only really dominating because there aren't any major players.
Some really, really great innovation happening in this space, though, and the competition Microsoft would bring would only be a good thing, primarily because there's really nothing to leverage. HPC is about pure, raw computing power.
Microsoft's really managed to do that well on certain applications (SQL Server, for instance), so I'd hope they can take some lessons learned from those groups and apply them to HPC.
I looked at some of the job openings for the HPC group, some of the guys here actually seem quite suited for them (don't want to name names).