I've actually had the same experiences as the above. Never flatted yet on my Pro3 Races on my road bike, well over a thousand miles since June on those. However I've flatted 6 or 7 times in the last 2 months on brand new Kenda Kwests in 700x40C on my cruiser.

I've found the same types of glass shards that go right through the Kendas and poke the tube flat, safely nestled in the rubber of the Pro3 without making it through. My theory is that it's the higher tire pressure in the road bike tires (110psi, vs 70psi in the cruiser) that keeps them from getting through. I use the same brand/model of cheap tubes in all my tires, too (SunLite), the correct size for each tire. You'd think the heavier, thicker commuter tires would resist flats much better, but that's DEFINITELY not the case for me... they seem to pick up every little shard of glass and send it straight through the tube.