Jon,
Here's a diagram of the setup we used for the linear bearings/elevator on our 2005 robot. (See Ben Piecuch's post.)
Linear bearing "a" was attached to the back (non-moving) 80/20 member, while linear bearing "b" was attached to the bottom of the elevator extension. Here are a few pictures of the linear bearings and our elevator. The first two pictures are when we were working on the robot, so many many of the protective panels were removed from the 'bot. (Click to view larger versions)
Good luck,
Art