From a quick google search, this website link came up:
http://www.lalecheleague.org/llleade...nJul99p64.html
It basically says that piercing will not have an effect on your ability to breastfeed, but two points I thought were noteworthy:
1. They reccomend a healing time of 10-16 months, so most piercers will not pierce a pregnant woman.
2. The only time there seemed to be problems with the piercings is when the mother tried to breastfeed while leaving the jewelry in. That's dangerous since the baby can choke, and they seem to have problems latching on and sucking.
I have not had any personal experience, but the article seemed to be pretty well documented with research.