A Princeton computer scientist team has released a white paper which documents their cold boot attacks on encryption keys. The scenarios are based on a key recovery involving primarily laptop computers using various methods to keep the key stored in dram on a power off.
Very interesting article and a guideline on preventing attacks like this. The most effective way is certainly limiting boot options in order do prevent anyone booting up tools to steal your encryption key.
http://citp.princeton.edu.nyud.net/pub/coldboot.pdf
SecuriTeam Article