I wanted some wetsuit socks rather than boots, to go inside my wakeboard bindings for winterbwakeboarding. Came across these, and they've been highly recommended. Bought a pair today, then thought they would be amazing for wearing under bike boots if you're riding in the wet and your boots are anything but 100% waterproof. You might still get wet feet, but if they keep me warm swimming in 3 degree lakes, they should work well on the bike too. They're 2mm neoprene, so not too thick.
http://www.powerkiteshop.com/oneill_heat_socks.htm
Anyways, worth a look. I'll give a fuller report once I've been out on the lake this weekend!