A recently released demonstration from Princeton University shows that most disk encryption systems, including Microsoft Vista Bitlocker, Apple FileVault, Linux dm-crypt, and TrueCrypt, are vulnerable to a simple attack that will result in the attacker being able to read the entire contents of an encrypted hard drive.
Lessons learned:
It is still highly important to prevent physical theft of a laptop computer
It is preferable to shut down a system as opposed to leaving it in sleep mode