This has come up in the past couple of weeks, being the fact that I do work as an IT Security Analyst, this is something you should know about laptop hard drive encryption. Mostly you may be using this in a business/work environment in order to protect sensitive data (oh how we hear about how laptops get stolen and their full of SSN numbers) or you may have this already set up on your personal laptop.
But now it appears there are now ways to circumvent the encryption, the link provided is a description from what researchers at Princeton were able to do.
Hackszine.com: Extracting encryption keys after a cold boot
Not hard to do.....