Or even something simpler - call the account holder's phone and play a simple recorded message that says "hey, its Rackspace, we need you to deal with an urgent issue, please check your email ASAP". Cheap for Rackspace, gives the customer fair warning, and if there's no response within some reasonable time frame then shutdown the machines in question.

What if I'm on a two-hour flight, though? A one-hour response time is ridiculous.

Well I didn't agree or disagree with one hour, I said "some reasonable time frame" explicitly because the expected response time needs to consider what the real risks are.

What if I'm asleep in bed?

You don't answer your phone if you're in bed?

I keep my phone permanently on vibrate (so I don't have to remember to silence it when in class, etc.), and besides, it'd take a hell of a lot more to wake me up than a ringing phone.

Some of us are heavy sleepers. I often manage to sleep through an alarm that I can hear through a brick wall when awake.