Since the script depends on two other ruby gems (namely crypt and highline), I've uploaded it to rubyforge. You can now simply install the script with
gem install keychain
The interface is quite easy, you can either pass the command as first argument, or invoke keychain which then enters an interactive mode. Simple try
keychain helpto see a list of commands.
Needless to say, I cannot guarantee that this tool will not one day forget all your valuable passwords, but at least I can assure you that I'm personally trusting my code. :)