On dpreview, when they reviewed Sigma's 10-20 lens they mention this: "Further good news is that normal 8mm-thick polarizers can be used perfectly happily without vignetting, and the slim-mount type is not required."
I was told the same thing by a salesperson @ CP: a normal CPL won't produce vignetting as long as I don't stack it on top of another filter i.e. UV. Is this true? FWIW I have a Canon EF-S 10-22.