MS seems to be taking Ballmer's 'developers, developers, developers, ...' line very seriously. .NET was a toe dipped in the water, and they found that by opening .NET to all platforms and providing copious good documentation they got the support of a lot of software devs. .NET is now an attractive platform for many tasks. XPS looks to be the next logical step: an open document standard based upon XML, which is one of the more ridiculously popular buzzwords. Documents stored as XML are going to be huge, but on the other hand, text formats do compress very nicely.
It seems they're learning that releasing correct and detailed documentation for their products does not lead to their technologies being reverse-engineered and stolen (hell, WINE has shown that bad docs and obfuscation are no barrier to that anyway). So now they're being as open as they can be, to see how that works out for them. Admittedly, their stuff wouldn't be so popular now unless they already had a huge market base...
A paranoid conspiracy nut might suggest they had this in mind for the past twenty years (using whatever means necessary to get a large user base, then switching to better practices while in a position of dominance). I personally doubt anyone can look that far ahead.
