Ha, i'd just written to buy a CPL in your other thread. The Hoya HD-series are 'high-transmission' CPLs, Tiffen make them called Hi-Trans, the B+W versions are called Käsemann. They let a lot more light through than a normal CPL, so you can get higher-shutter or lower-iso compared to a normal CPL. I keep meaning to get one for my 70-300L but i'm too lazy.

and Wow, I just read your post again and realised you said 15-85 not 18-55. That was a good deal on the body, but with that lens that's almost an insanely low price. I paid $2500 or so equivalent for the same brand-new a year ago.

Anyway, the 15-85 has a 72mm front-thread, and the 70-200 has 67mm. My tip would be get the 72mm version, get a 67-72mm step-up ring (like $5 on ebay from china), and use the same filter on both lenses (although you may not be able to get the hood on and off, I have that problem with my 70-300L, so leave the hood off when I use the CPL, which is most of the time).