Here in southern california, I have bike commuted nearly every work day for the past 5 or 6 years in sandals. I average about 25 miles round trip per work day unless the weather is really bad and I take a short cut to make it a 14 mile round trip. I pedal free footed, on MKS Lamda platform pedals (not clipped or strapped to the pedal). This year I've been wearing Nashbar Ragster sandals (about $45 at BN) with whatever combination of socks that keep my feet comfy. Socks and sandals, of course.

In warm temperatures I wear sandals with a pair Smartwool light hiker socks, because they help keep my feet from feeling sweaty against the rubbery waterproof footbed of the sandal. At this time of year, morning temps are typically mid to upper 40s F, so I wear a smartwool thin liner sock under a smartwool regular thickness hiker sock. Works great. If the temp is 30s to low 40s, I wear two pair of smartwool regular thickness hiker socks. I don't know what sock sandal combaination is good below 32F because temps have not been that low here in the past 12 years. Multi layer socks should not fit super tight, they should be pretty easy to pull on and off. The Nashbar Ragster sandals have adjustable Velcro bands, so they work fine with variable thickness of sock layers. If it is actually raining, I switch to sealskins waterproof socks, with a thin smartwool sock underneath to control sweat inside the sealskins. These things work great, and I have used that combination in heavy, cold rain downpours where the feet stay reasonably comfortable.