I just completed backing up 8 years of my Gmail using http://gmvault.org
I selected the --encrypt option which uses Blowfish encryption. According to their site:
Emails can be encrypted with the option -e --encrypt. With that option, the Blowfish encryption is used to crypt your emails and chats and the first time you activate it, a secret key is randomly generated and stored in $HOME/.gmvault/token.sec. Keep great care of the secret key as if you loose or delete it your stored emails won't be readable anymore !!!
I'm using OSX Lion. I'm a software engineer but far from an encryption expert.
What should I do with this key? It seems like leaving it where it is now (alongside the emails) sort of misses the point of encrypting them to begin with.