Basically, the pilots need to get their masks on and get the plane down below
25,000' as rapidly as it can possibly be done.

A couple links to the sad story of Payne Stewart a few years ago. I was
working in Minnesota and there was great consternation as the plane autopiloted
its way across a corner of the state and the military wondered if it would be
necessary to shoot it down. Had it come much closer to a big city, they would
have had to make that decision.

http://www.newsmax.com/articles/?a=1999/10/25/164032

http://www.cnn.com/US/9910/26/shootdown/

The NTSB report:
http://www.ntsb.gov/publictn/2000/AAB0001.htm

I'd always assumed, as many did, that something catastrophic happened shortly
after takeoff, like a window failing, and the pilots didn't even have time to
get oxygen masks on. A friend suggests it never pressurized, which makes one
wonder whether they could become anoxic to the point they were helpless even
without a catastrophe, if they didn't realize the cabin was never
pressurizing...but lots of folks have told me about the foolishness that comes
with lack of oxygen. The full report makes fascinating reading...accident
investigation includes lots and lots of stuff.