I agree with hikermor - as long as there isn't deep snow on the ground and I am hiking, no need to wear pants in the rain when shorts will do. The activity keeps me plenty warm as long as my torso is dry underneath a raincoat (of course then sweat becomes a problem).

If there is snow when it rains, generally I am wearing wool pants and capilene or polypro, which retain heat when damp and wet. But in this type of environment it is often better to hunker down and let the weather pass.