A cold-boot attack powers off a computer, then boots it to a special program that copies the memory contents to a USB drive. The hacker then scans the memory dump for the stored information and extracts disk encryption keys.
To protect your equipment against these attacks, exercise good physical-server security and disable the ability to boot from a USB device. This protection won't stop an attack, but it will make it more difficult. If an attacker physically has a box, he or she can power it down, remove the RAM, and put it in another box (unless you solder the RAM to the motherboard). Always power down laptops—don't leave them in sleep mode. Using a Trusted Platform Module (TPM) won't help because the TPM initially stores the key, then puts it in memory for decryption.
A Princeton University video of a cold-boot attack is available at http://www.hackaday.com/2008/02/21/breaking-disk-encryption-with-ram-dumps/. The companion paper is at http://citp.princeton.edu/pub/coldboot.pdf.