second vote for REI's Acme pant ($165 bucks and worth every penny); they're one of those products that are near duplicates of Big Name's stuff that turn out even better. I beat the crap out of mine and they keep going; rock, ice climbing, snow-shoeing etc.
BUT they're not actually water-proof, just resistant. So i'd also put forward their Taku pant (60mph windproof and waterproof), still very flexible and lightweight, breathe very well and solved all of the problems associated with their models from the previous two years.
Both pants are worth their price tags - they will last years, even with heavy use