Had the tire been filled with air, rather than CO2? CO2 leaks out through the tube, so tires inflated with CO2 will go flat after a while.

As an aside, on my commute and utility bikes, I use Schwalbe Marathon Plus tires - not the lightest and fastest of tires, but when you count the time saved in fixing flats, they seem much faster. I have (between the two bikes) ~13,000 flat free miles on SMPs. One disadvantage is that you forget how to fix a flat ;-P