Several media outlets are reporting receiving cryptic invites to a "mysterious" Microsoft media announcement on Monday. No word on what it is about, but what has been mentioned is that it is "major", "big", shouldn't be missed, etc.
Nothing but speculation right now, but some think this could be the announcement of the next Xbox, which would indeed be major. MS wouldn't have announced a new console at E3 given they woudn't want to share the stage with anyone.
Here's to hoping that this is about the new Xbox 720/Infinity/insert name of your choice.