I find a pair of waterproof/windproof track pants good for the wet and cool weather we're having. I have a Goretex pair that are great for warmer weather when you sweat a bit. The nylon ones are ok for now. Look for lined ones for colder weather.
You're right Boudicca, a helmet cover is the only thing that'll keep your head dry, but they surprisingly keep your head warm, too. I wear a Louis Garneau baliclava under my helmet when it's a little cooler out (around +3 or so), nice and thin, but very warm.
My first fall commuting 3 years ago, I became hooked when I realized that I could ride comfortably in some of the most uncomfortable weather I could imagine (temp of +3 and raining!)