I've used the Col d' Lizard tights (
http://www.coldlizard.com/cgi/wc.dll...alog~DETAIL~34) in cold rains, snowstorms, etc. without discomfort. They only go up to XL, but they're extremely stretchy. There's a review at
http://www.icebike.com/Clothing/Ctestedonice.htm if you want to check it out.