I read what Gordon said about polarizing filters with his photos from the bolivian altiplano and I remembered that I hadn't tried this filter with my 40D. I used it the last time on my Rebel K2 film camera and the results were variable since the 28-90mm USM had a rotating front element and it was a pain in the neck to make it work.
Today I tested it and here are the results.
f4 @ 28mm
f11 @ 30mm
Auto levels on both with CS3
No color editing