The zippers are weatherproof zippers and the material is supposed to be waterproof. That said, for cheap you can buy a backpack cover ($15 or so). I always use that too and that's what I'm going to do with mine. They are light and take up little space.
I got mine - also in read. Very impressed so far. It carries really well and has a lot of room for other stuff. I was able to put an extra layer (down jacket) in there, an extra pair of guide gloves, extra hat, my TT belt and a couple of lens changers, cotton carrier, my misc stuff (cleaning stuff, filters, etc...), a 70-200 f/4L IS,300 f/4L IS 1dmk3, 24-105 and an extra 40D body with grip. I still haven't gotten the ICU configured just right, but it will work fine once I futz with it some more.
Interestingly enough, it is also possible to remove an item from the ICU without taking the pack completely off. Loosen the waist belt, take off the shoulder straps, and rotate the pack around in front and lay horizontal supported by a knee. Open the ICU< get what you want, and then zip it back up, swivel around to the back, put on shoulder straps and tighten waist belt. Ski away.