I suppose you could put it in a safe deposit box. Another option is hide it in the margin of a book; somewhere that normally wouldn't get stolen or read. Of course then you have to worry about fires and floods... Perhaps a copy at a relatives house as well as yours. For bonus points, don't tell the relative, just jot it down in a book on a shelf when he/she isn't looking!

It was largely rhetorical. :P A lot of the comments on this are related to how do you keep important data both always available to you, -and- safe from anyone else getting ahold of it. I was just pointing out the bootstrapping issue; having one password/phrase/key that allows you to reset the passwords/whatever to your digital life still requires you to find a way to keep that one word/phrase/key safe yet accessible.