When I click over to my reserve tank my bike ('92 GS) stalls out.  After 15-20 seconds I can get the bike started again, but boy this sucks because I commute on an expressway.  Does anyone have any ideas what is going wrong?

The floats are empty and it takes a few seconds to fill them up since the reserve valve is open only with vacuum.
Turn the valve to PRI for a few seconds, then to RES. That should you get you going faster.

Is it possible that your fuel lines are reversed?  If you are using the RES fuel line when the petcock is in the ON position, then your REServe has already been exhausted by the time you switch to the RES position (using the ON fuel line).

Maybe it's just a question of switching from ON to RES quickly enough, like miket hinted about.  Are you able to do it while scooting down the road?  (I put the back of my left hand somewhere along my left calf muscle, and that puts my fingers right on the switch.)

But just for kicks, make sure that the front fuel line from the gas tank hooks up to the rear of the ON/RES/PRI petcock, and vice versa.

Sometimes you'll get air stuck in the Res line if it gets too low and is unused. I've experianced this a couple of times. One way to avoid it is to change the petcock to reserve for a little bit after filling up the tank to let fuel in and air out, or even taking it through all 3 positions a few times. This doesn't always make a differance but I haven't found it to hurt.