We've reported extensively on an issue in which Late-2008 (unibody) MacBooks and MacBook Pros freeze when waking from sleep. We previously recommended that users not connect or disconnect any external devices while the new MacBooks are asleep, and suggested that users may want to disconnect any external devices before putting the machines to sleep.
It now appears that this issue does indeed involve external devices, specifically Bluetooth devices. Several users have now reported that turning off the "Allow Bluetooth devices to wake this computer" in the Bluetooth pane of System Preferences fixes this issue.
One MacFixIt reader writes:
"I note the freezing when I have left Bluetooth running with a cordless mouse when I put the book to sleep. It almost seems as if it can't find the device, it hangs, but that's just a guess, and it only happens intermittently, not all the time."