YMMV - but I've had small sharp things hurt a little, like sharp bits of gravel, and never had anything pierce the sole. I've also ran over small amounts of broken glass by the side of the road before, no problem.
The soles are really strong, but I wouldn't rule out a perfectly aligned spike or blade (scalpel tip or something?) going through -- but to be quite honest, something like that would probably damage a traditional running shoe; protection isn't a substitute for looking where you are going.
The soles are really strong, but I wouldn't rule out a perfectly aligned spike or blade (scalpel tip or something?) going through -- but to be quite honest, something like that would probably damage a traditional running shoe; protection isn't a substitute for looking where you are going.