Some further information. I see that I might have gotten carried away again. This happens. After poking around the web for an hour, my own conclusion is that breastmilk itself doesn't cause cavities, but that some steps are in order to keep infant teeth healthy. These include (1) Dental checkups for parents and filling of any cavities they have (2) Wiping teeth after feeding, especially at night (when the saliva provides less resistance to the bacteria that cause tooth decay) (3) Not feeding bottles of sweetened juice or smearing pacifiers with honey (4) Fluoride supplements. But I'll let you make up your own minds. Here's what I found:
http://www.parentsplace.com/expert/lactation/special/qa/0,3459,6365,00.html Mentions both archaeological and modern evidence ("Dr. Palmer also found very little decay in ancient skulls and has concluded from his extensive research that breastfeeding does not cause tooth decay."). Also notes that pregnant women should have their own teeth checked and any decay repaired, as it's theorized that the responsible bacteria may travel from mother to baby. Includes a good list of scientific references that you can point your doctor at so he knows you're not just getting this from militant nursers. (Not that I think nursing moms know less than researchers, but I didn't go to med school!)
http://www.bflrc.com/ljs/myths/dentalca.htm Debunking of the most-cited study that suggests a connection, again with references from the medical literature.
PubMed (the National Library of Medicine database) cites a journal article from a 1998 issue of Caries Research (there's a magazine for everything!) Conclusion, based on a study of Dutch children who were breastfed on demand for 2 years or more: "The results of the present study demonstrate that prolonged demand breast-feeding does not lead to a higher caries prevalence although comparison between the groups demonstrates that frequent breast-feeding and low additional fluoride use should be considered as contributing factors in the process of nursing caries."
The University of Washington School of Dentistry has a page of information on pediatric dentistry here: http://www.dental.washington.edu/pedo/Drinfo.htm. Again, nothing pro or con on breastfeeding, though they do mention that babies should have breast milk exclusively for six months. They talk about a WIC program to prevent BBTD by handing out cups when children are six months old, and oral mirrors so parents can keep an eye on the kid's teeth. They also make the point that an exclusively breastfed child is not getting the benefits of fluoridation in the water supply and should get fluoride supplements starting at 6 months (assuming you believe in the benefits of fluoridation; I don't really want to argue about that just now).
The American Dental Association's page on BBTB is at http://www.ada.org/public/topics/bottle.html . They send mixed messages, depending on which links you follow. Some emphasize only avoiding nursing on sweetened juices from a bottle and smearing pacifiers with honey (which none of us would do for an infant anyhow!). But elsewhere they advocate weaning "as soon as he can hold a cup".
A Washington Post article says babies put to bed with a bottle of formula or juice are at greatest risk. But, they add, "Children who breast-feed after their teeth have erupted can also develop dental decay if their teeth are not wiped clean after each feeding, especially at night. Saliva, which helps eliminate the acid, is produced in smaller quantities while sleeping."
A 1998 paper from International Journal of Pediatric Dentistry reports (based on a study of 200 Swedish children at age 18 months) "The results indicate that Swedish children with prolonged breast-feeding have a tendency to establish unsuitable dietary habits which constitutes a risk situation for developing caries at an early age." Note that they found the risk factor was not the breast milk itself, but that these children "had significantly higher numbers of cariogenic food intakes per day" than those without decay.
A 1999 article in Pediatric Dentistry concludes "It is concluded that human breast milk is not cariogenic." However, this appears to have been based largely on soaking extracted teeth in breastmilk, not (in my opinion) a very realistic simulation of the situation in an infant's mouth.
The American Academy of Pediatric Dentistry says flat out, "Don't nurse your child to sleep". Their brochure of information for parents is at http://www.aapd.org/parents/broch1.html . I can't find anywhere on their site that they list research references for their positions, but it's probably a good idea for parents to read this sort of thing to see what their medical practitioners are being told.
*** WARNING ***: Disturbing photos on this page: http://home.flash.net/~dkennel/bottle.htm . Another pediatric dentistry site, this one shows what Baby Bottle Tooth Decay can look like. Recommends early weaning from the bottle. Doesn't say much of anything about breastfeeding.
Not normal kids, but there's a bit on eating disorders including inability to tolerate solid food in some young kids in a Seattle Times story.