If you happen, like I did today, to drive through the main Microsoft campus in Redmond you will see a big tent on their main soccer field along with a stage being built on their softball field with lots of audio being set up. Looks like they are gearing up for a big party for their guys and gals that work on their OS as that seems to be the only time they take over the playfields like this. In fact back with Windows 95 shipped they totally destroyed the playfields with the party they had and it took months to re-seed and plan new grass. They now have the nice field turf that you find in any major stadium and that will never die, of course.
Update: Looks like it might have been an intern fair or something for the hundreds of college and highschool interns that they hire. Windows Vista might have had their party some place else.