"Evil Maid" Attacks on Encrypted Hard Drives
Earlier this month, Joanna Rutkowska implemented the "evil maid" attack against TrueCrypt. The same kind of attack should work against any whole-disk encryption, including PGP Disk and BitLocker. Basically, the attack works like this:
Step 1: Attacker gains access to your shut-down computer and boots it from a separate volume. The attacker writes a hacked bootloader onto your system, then shuts it down.
Step 2: You boot your computer using the attacker's hacked bootloader, entering your encryption key. Once the disk is unlocked, the hacked bootloader does its mischief. It might install malware to capture the key and send it over the Internet somewhere, or store it in some location on the disk to be retrieved later, or whatever.
Via Gust MEES