Reader Chris Williams is studying a PhD in mechanical engineering. He's also a man with a theory:

I thought you guys might be interested in a little weekend project of mine. I fabricated a clip that attaches the Wiimote to the Classic Controller that uses the "mysterious" 3rd shoulder button.

I've always hypothesized that the "mystery" button (un)connects an as-of-yet unreleased clip that would allow you to attach your wiimote to the Classic Controller. The idea being that such a thing would be good for (a) getting the wiimote out of the way and (b) for any games that may want to have a mix between dual-analog controls and motion-sensitive controls.

You can see where this is going. Williams crafted himself a clip to join the Wii Remote to the Classic Controller, and it looks none. Too. Shabby. Now all he needs is a game to actually take advantage of his handywork. Luke Plunkett

[Thanks Chris!]