^
I've had similar experience, in picking up debris in the tread. It seems like that's the only time I get flats, and I can't really feel too bad about it considering how long the little piece of glass or metal probably had time to bore a hole into the tire.
Curiously, I've never flatted on the slick tubulars that I run on my Sutter, but that bike came with wire gaurds that attach to the brake bridges and brush off any debris that might get stuck in the tire. The wire gaurds wouldn't work with a treaded tire because of the resistance the tread would create on the wire.
By no means am I advocating tubulars for touring, though. But the wire guards, I could see being useful for those who use slicks. I doubt they're available any longer though. They create a little resistance, that can be heard if not necessarily felt by brushing the tire.