I wear mine barefoot at this point. I have a pair of classics and a pair of sprints. I have a pair of Bikila's on the way (for the colder weather) and will probably wear Injinji socks with them. Most of the e-tailers who sell Vibram's sell the toe socks as well. I think any toe socks would work though. You definitely couldn't wear regular socks with them.
Carrie