The cold boot technique is a technique pioneered at Princeton a few months back that can be used to recover among many other things the keys used to do whole disk encryption.  The team at Princeton has a good video explaining how the technique works here: http://citp.princeton.edu/memory/.

The technique is actually pretty straight forward, rapidly terminate a running (or hibernating) machine  and get it to boot your usb thumbstick asap (this can also be done over the network via PXE).  Once the thumbstick boots up with their tiny piece of software, the software simply dump all memory to a file on the usb stick. Once complete you are free to go to any other machine  with your thumbstick and analyze this dump file with any of your favorite forensic tools. Many treasures can be unearthed by analyzing this dump but to make it easier to find AES and RSA encryption keys in the memory dump, the team also provides 2 tools for the job. The whole disk encryption key is only one of the many thing an attacker will have access to after having access to a  memory dump.  All running programs, lmhash passwords, files in memory, etc will be recoverable using common disk forensic tools.

Last but not least, and perhaps the most interesting aspect of their research is that they found out is that they were able to make the memory stay around in RAM for up to 10 minutes at a time by cooling down the dram chips in the computer before abruptly terminating it.  In their demo they were able to use common dust spray cans to cool down the memory sufficiently. Their numbers were for warm DRAM chips were more around the seconds to minutes range depending on many factors. It maybe a good idea to test your machines to see how much data you can recover from memory after a cold boot so you can determine how vulnerable they are to this technique.

To mitigate this risk you should layer encryption of very sensitive documents  by using file or virtual disk encryption within your encrypted drive.  Remember that whole disk encryption should only be one of multiple layers,  reduce the risk of this physical threat by combining whole disk encryption with  physical security measures. Shutting down the machine completely when possible may also be a good idea.