My son was biting at that age too. It was the kids he liked a lot and me. I tried it all. Spanking, biting him back, time-out, etc. (He has a VERY high pain tolerance, so pain based punishments do not phase him).
What did work was to tell him how "sad" I was AND soap!!!! I started out with ivory, now I grab the liquid stuff and he really does NOT like it. His teachers would tell him that "Mommy will be sad" and that he was about to get soad and that stopped it - FINALLY!
Other options are vinegar and hot sauce. Well vinegar is really not that easy to carry with you and he likes hot stuff but it may work for you.
I found that with both my boys, the biting was tied to communication and the lack of ability to communicate their positive and negative feelings. They will grow out of it but I know how it is to dread the preschool teachers and directors telling you - he did it again! GOOD LUCK!!