I've found the answer! How to separate out the on-off switch:
The circuit board is fixed to the base/bottom of the mouse by one silver screw in the center--you can see the screw clearly in the last photo Darren posted. Remove that screw, and lift out the circuit board. Since the on-off switch is just sandwiched between the circuit board and the mouse's base, it will fall out freely as you do this.
But but but I *like* the click! 0.0 Seriously, though, it's really cool that even *that* is user-serviceable. It's amazing how accessible this piece of equipment is.
(I wandered back here today because the left button had gotten suddenly unreliable, and I needed to remind myself how to take the mouse apart. Once again, cleaned out the dust and everything was fine. Must remember not to leave mouse in the same room where sawdust is flying...)
While I'm commenting: Darren (or anyone), do you have any tips on separating out the on-off switch? The raised "crossbar" texture has gotten so worn down that I can no longer switch mouse on or off with a fingertip; I have to dig a fingernail in there, or the tip of a tool. I'd like to give that switch a couple coats of nail lacquer or polycrylic or similar, to build the texture up so that the switch is fingertip-operable again. But I don't want to try it while the switch is in its little oval recess and risk gumming up the works.
...I really hope that made sense.