GH, pathetic? There has been a great deal of research done in the area. In my personal opinion there is not yet any really conclusive evidence that there is a robust link, although some very persuasive studies suggest one, but by the nature of the phenomenon studies are necessarily observational.
If anyone wants to review some of the evidence for themselves about the outcomes of breastfeeding in developed countries such as ours, I found this systematic review to be helpful and straightforward - I have linked to the abstract, as I don't want to violate copyright by linking to the pdf of the full article. It's here. Bear in mind that more research will have been published (and will still be underway) since it came out. However, it is a thorough, well-conducted and evidence based piece (they looked at 9000 studies in the area).
For those that do not want to click on the link, here is a synopsis of the key findings:
We found that a history of breastfeeding was associated with a reduction in the risk of acute otitis media, non-specific gastroenteritis, severe lower respiratory tract infections, atopic dermatitis, asthma (young children), obesity, type 1 and 2 diabetes, childhood leukemia, sudden infant death syndrome (SIDS), and necrotizing enterocolitis. There was no relationship between breastfeeding in term infants and cognitive performance. The relationship between breastfeeding and cardiovascular diseases was unclear. Similarly, it was also unclear concerning the relationship between breastfeeding and infant mortality in developed countries. For maternal outcomes, a history of lactation was associated with a reduced risk of type 2 diabetes, breast, and ovarian cancer. Early cessation of breastfeeding or not breastfeeding was associated with an increased risk of maternal postpartum depression. There was no relationship between a history of lactation and the risk of osteoporosis. The effect of breastfeeding in mothers on return-to-pre-pregnancy weight was negligible, and the effect of breastfeeding on postpartum weight loss was unclear.