Same way that you do it now - copy the relevant files out of ~/Steam/steamapps/ to wherever you need. I've gotten around re-downloading huge games because everything you need is in there (when I've already done it once before on a different machine, or when I'm getting a game a local friend of mine already downloaded). The only thing stopping you from playing it is the DRM on the executable, but the mythical Steam-is-dead-here-you-go-patch will solve that.
