Update 11/3/2008: The problems described in this post were resolved in Windows Vista Service Pack 1
I’ve done some experimenting to find the cause of my ReadyBoost problems. I found that the drive always hangs after the computer comes out of standby. After the computer wakes up, the thumb drive works for a little while and then hangs. The hang is not recoverable and prevents the computer from shutting down.
The solution I found is to select the drive in Explorer and choose Eject. It is not possible to choose Safely Remove Hardware because the device is in use. Then I pull out the thumb drive before putting the computer to sleep.
When the computer wakes up, I put the thumb drive back in. Sometimes Windows automatically starts using the drive again for ReadyBoost, but usually I have to go to the ReadyBoost tab in the Properties dialog and reenable ReadyBoost.
I've sent a technical support request to Apacer but have not heard anything back.