Given that Microsoft have invested lots of time and money in their own web server (IIS), this is a true endorsement of changes afoot at Microsoft http://news.bbc.co.uk/1/hi/technology/7528857.stm
I personally think this makes complete sense for them. I truly believe as we move to a more service delivered software models, the value of source code (or software IP) becomes less important, as licence revenue becomes less of a percentage of total revenue for the big boys such as MS.
For companies like Microsoft, interoperability is very important and its relevance is definitely growing. For example, I am sure that to allow users to be able to transfer services currently running on a LAMP architecture to run on Microsoft's own hosted Windows "cloud" architecture is going to be important for them in future (even if it's not running on a .NET platform). Here is some more detailed info
Interestingly enough, their competitor Apple seems to be going in the other direction with their iPhone SDK, for which the licence agreement apparently does not allow people to build open source software... hmm!