Ambien is a small kernel extension that prevents the system from sleeping only when the lid is closed. It was written because Insomnia would prevent the system from sleeping always, not just when the lid was shut; it was named Ambien because it solved my problems with Insomnia.

I should note that disk I/O can get pretty slow if you're carrying the machine around because the disk's accelerometer will still put the drive to sleep! If you're using VMware, your virtual machines might time out disk accesses; to solve this, try the following (inside the VM):

echo 360 | sudo tee /sys/dev/block/8:0/device/timeout

(Thanks to Michael Roy at VMware for helping me diagnose this!)