Link: http://en.wikipedia.org/wiki/Cold_boot_attack

Local attacks against disk encrytion.

Bruce Schneier blog

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.

Interestingly, if you cool the DRAM chips, for example by spraying inverted cans of “canned air? dusting spray on them, the chips will retain their contents for much longer. At these temperatures (around -50 °C) you can remove the chips from the computer and let them sit on the table for ten minutes or more, without appreciable loss of data. Cool the chips in liquid nitrogen (-196 °C) and they hold their state for hours at least, without any power. Just put the chips back into a machine and you can read out their contents.

source and full text:
http://www.schneier.com/blog/archives/2008/02/cold_boot_attac.html

The following video shows how to do it:

You may also want to check the following:

Venona project

Truecrypt linux encryption and cold boot attacks