So sit back, have a glass of wine, and enjoy your Saturday night. Try not to use the AH, because finding all your stuff gone wouldn't be cool (you'll get it back though, I'm sure, if it is gone, one way or another).
We'll update this post when a resolution is announced.
Update: Rolling restarts are being issued to correct the problem. For servers which have been restarted, the issue appears corrected. The opening announcement on the loading screen makes note to say that no items will have been lost during this AH outage.