Lizard Skins makes a headset seal, it is just a piece of neoprene with some Velcro but it does a good job of keeping the gunk and water out of the lower bearings and is only a few bucks.

I use loose ball hubs (Deore LX) on my wet weather commuter and repack the grease at least once a month. I live in Vancouver Canada and in the winter often hit puddles where my hub is nearly submerged so the hubs get water in them pretty quick. I also like to go with a plated chains and cassettes (I use SRAM PC-68 chains and 850 cassettes) to try to keep the rust to a minimum.