Wouldn't there be contention issues around writes to EBS in this case? I know very little about this, but I keep hearing about EC2's relatively poor IO performance and imagine that there would be a big fat traffic jam if every running instance received a signal to get their house in order before filesystems are forcibly unmounted.
Absolutely, and that seems to be a big part of why EBS takes so long to come back - simply booting all those disks over the network generates so much traffic it takes forever!
On the other hand, an fsck to recover the filesystem probably causes even more traffic.
