I have actually worn my Keen bike sandals in high 40's with performance DeFeet thin socks and have been quite comfortable. The bike sandals have a front rubber "bumper" that blocks wind, and the socks are densely woven. Your toes MUST have wiggle room to keep warm .: thin socks allow more room inside the shoe.
Usually below 50F I will wear my SPD MTB shoes and thin socks.
For much colder days, I have jerry-rigged some Glad "Press-n-Seal" wrap over the socks in the toe/forefoot area ONLY (inside my MTB shoe).
My bike commute involves walking over some rough terrain for about 100 feet getting across some train tracks. That stretch would tear up booties or toe covers pretty quickly and I need the good traction that I get from the MTB shoes/sandals.
For even colder days, I put platform "campus" pedals on - SPD cleat on one side, rattraps on the other. My thinking was that I could wear warm boots. Well - it never got THAT cold (below 25F) here and I hated the pedals (not as easy to clip in as my others). I also bought some SPD "inserts" that convert a SPD pedal to a platform + toeclip cage. Never used those either. After slipping off the pedals on damp days, I am a complete convert to clipless. But the platforms/inserts are options you may consider, could work for you.
Last edited by nkfrench; 10-25-09 at 07:56 PM.