Usually I try not to just reiterate what is written elsewhere anyway. But this thing is so hot and amazing that I just have to provide you with a link: New Research Results: Cold Boot Attacks on Disk Encryption.
In a nutshell: Hard disk encryption master keys are stored in DRAM. However according to the research results of Princeton University, DRAM does not lose its content immediately, but rather only after a couple of seconds or minutes. If cooled (e.g. by spraying cooling spray on it), memory content can be preserved for multiple minutes and restored after booting from a malicious operating system.
I strongly recommend you read it for yourself.
Picture of a Hard Disk by cgommel
Cyberphobia 