Sam2- As this is a hot topic now- especially because a Rabbi in Monsey was quoted in the Journal news (see news section) , so I did some research. The gemoro and the Rambam (quoted in this thread) talk about “metzitzah’ but do not add the word “be-peh’The Hebrew word “metzitza” seesm to indicate by mouth, although this is not conclusive. From a quick web search I found a site that deals with this and there, it claims that the first time “metzitza be’peh’ is mentioned’ is by the Remo and subsequently by the Kabbalists. That makes it fifteenth/sixteenth century onwards.
The most intruiguing aspect on that site is the claim that the Syrian, Yemenite and other Sefardi jes do NOT practice “metzitzah be-peh”. That would make it a European minhag, almost exclusively (yes- I know the Ari was a sefardi). If any psoters know anything asbout this, please inform us!