Probably one of the better questions I have seen posted -=)
By definition is the implants were used to replace missing or removed breasts then yes they would be technically, if you consider them a limb.
Definitions of Prosthetic limb on the Web:
* In medicine, a prosthesis (plural prostheses; from the Greek ????????? "addition") is an artificial extension that replaces a missing body part. ... en.wikipedia.org/wiki/Prosthetic_limb
For vanity reasons then I would say no.