I've had to disable ReadyBoost on my system. I've been using the Apacer HT202 Handy Steno 1GB drive, which at a 200x transfer rate is supposed to be one of the best drives on the market for ReadyBoost. I've run this configuration for two weeks now and have consistently run into the same problem. After using the system for several hours with dozens of windows open, the thumb drive hangs. It's not clear whether the drive itself hangs or whether it's a driver issue, but Vista is not able to recover. Explorer hangs, the DOS prompt hangs - anything that tries to access the drive locks up and can't be killed in Task Manager.
A hardware reset brings the system back to normal, except now the RAID array is unhappy because it's been interrupted and it takes two hours to validate the mirror, during which time the system is unusable. (Intel ICH8R chipset on a P5b Deluxe WiFi motherboard.)
Hopefully a solution will present itself after Vista is released to consumers.