...and I've only been on two rides.
Even more frustrating, I can only figure out the cause of the second one (glass). The first was a complete mystery - just a little hole on the outer edge of the tube, but no apparent sharp or hole in the tire. The third ripped during CO2 inflation (a great big rip -anybody have any tips on that one? Did the rubber freeze and get brittle or something?) and the last one is again a complete mystery - a tiny pinhole without any obvious corresponding problem with the tire.
Screw it - the tire has about 2500 miles on it - I'm headed to the LBS to get a new one.