Ed Felten writes on Freedom-to-Tinker:

Today eight colleagues and I are releasing a significant new research

result. We show that disk encryption, the standard approach to

protecting sensitive data on laptops, can be defeated by relatively

simple methods. We demonstrate our methods by using them to defeat

three popular disk encryption products: BitLocker, which comes with

Windows Vista; FileVault, which comes with MacOS X; and dm-crypt,

which is used with Linux.

The root of the problem lies in an unexpected property of today's DRAM

memories. DRAMs are the main memory chips used to store data while

the system is running. Virtually everybody, including experts, will

tell you that DRAM contents are lost when you turn off the power. But

this isn't so. Our research shows that data in DRAM actually fades

out gradually over a period of seconds to minutes, enabling an

attacker to read the full contents of memory by cutting power and then

rebooting into a malicious operating system.