Our son was bitten several times at daycare and then, to out horror, started biting other children as well! He was about 14/15 months old at the time.
The child originally biting was new to the centre and, apparently, fairly stressed at being there and so was quite aggressive with the carers and other children. My son (and several others) were his hapless victims initially, but then I think they saw how much attention he got from biting and what a good reaction the bitee gave they all thoguth they'd have a go. It was a bit of a biting epedemic there for a few weeks! All of these children were 13 to 18 months old.
At the centre, they isolated the biter (put in opposite corner of the room) and gushed all over the bitee, giving them attention and all the cool toys. At home with out son, if he bit us we immeidately said "No biting" calmly but firmly, and carried to his room, put him down, said another "No biting" and shut the door. he has a sliding door and could very easily open it himself. When he came out, even if it was immediately, we were never angry with him, but if he bit again, same story.
Within a couple of days he had slowed right down to really only biting Mummy and Daddy when he was angry and frustrated and within 2 weeks he had stopped completely and has never bitten since, he is now 26 months.
The parents of the other biting offenders at the centre tried the same thing at home and all of the children had stopped biting within 2 weeks.
Personally, I think the biting them back thing is just showing them that it's OK to bite....if Mummy and Daddy can do it....?