Ah yes, our old friend -- the Post-it of Unavailability. The Apple Store is down in the US, UK, Australia and presumably elsewhere as well.
Could be maintenance, could be something else (although it seems awfully early for the rumored Mac Pro refresh). We'll know in a few hours, give or take.
Update: The store is back up with no major changes; however commenters note that the previous days-to-ship product information is now replaced with simply "In Stock" except in the case of the Mac Pro. That may be aimed at avoiding further commotion when simple ship time fluctuations occur.