For those of you keeping track of such things, I wanted to let y'all know 
that the leak was due to a crack in the expansion tank.  The crack isn't 
visible upon inspection, but the tank leaks when it gets very hot.  It 
was the original tank.

REplacement wasn't too bad.  I used vice grips on the two hoses going to 
the tank, and then a baster to remove the coolant so it wouldn't be on 
the ground.  I made the mistake of putting the new o-ring on top of the 
old one (not realizing that the old one looks flat), but someone pointed 
out this as a possibility to me when I said that the temp sensor wasn't 
sealing properly.  A guy at the dealer said that the plastic housing on 
the sensor sometimes cracks, and that he always just replaces the 
sensor.  Though I didn't take his advice on this, he turned out to be 
quite helpful and showed me how to apply some vaseline to the edge of the 
o-ring so the sensor can be screwed in properly.  I think the sensor 
often breaks when someone tries to screw it in too tightly.  Apparently, 
it should be little more than hand tight.

Thanks to Joel, Tim, and others for the help along the way.  

david