Thanks everyone for the suggestions. I think I'm headed toward waterproof socks. There are quite a few options I'm finding, from sealskinz (suggested) to Cabela's house brand to more than I can count really. Most of the Sealskinz reviews seem to support them being a good choice for cyclists. I'm thinking that between the socks (some of which suggest a thin liner sock for wicking) and the am-fib shoe covers, I may be able to avoid dropping a bunch of money on some boots.