I noticed today that I had an EC2 development cluster running that I hadn't shut down from a few days ago. It was only a couple of instances, but even so, it was annoying. Steve Loughran had a good idea for preventing this: have the cluster shut itself down if it detects you go offline - by using your chat presence. You'd probably want to build a bit of a delay into it to avoid losing work due to some network turbulence, but it would work nicely for short lived clusters which are brought up simply to do a bit of number crunching. Alternatively, and perhaps more lo-tech, the cluster could just email you every few hours to say "I'm still here!".
I wonder how many forgotten instances are running at Amazon at any one time. Is there a mass calling of ec2-terminate-instances every month end when the owners see their bills?