You'll recall that in February security researchers found a way to break into encrypted hard drives by using a trick that relies on the fact that RAM dissipates its contents slower if cooled.
The research paper, by Princeton, the Electronic Frontier Foundation, and Wind River Systems was titled "Lest We Remember: Cold Boot Attacks on Encryption Keys" (.PDF
"). Over the weekend, timed to coincide with the Last HOPE hacker conference over the weekend in New York, the source code was published
to the Web.
Of course, this technique only works if the laptop hasn't already been turned off, which would have enabled the memory to fully dissipate. This probably isn't the method most people on your laptop, as it requires some degree of expertise, but are you still worried about someone breaking your encryption using this method? Turn off your laptop; don't have it go into sleep mode.
Watch a video demo of the method at work: