This report says breastfeeding has no effect on the shape of a woman's breasts:

....according to a first-of-its-kind study presented today at the American Society of Plastic Surgeons (ASPS) Plastic Surgery 2007 conference in Baltimore. “Many women who come in for breast surgery tell us their breasts are sagging, drooping or are less full because they breastfed,” said Brian Rinker, MD, ASPS Member Surgeon and study author.

As the first study to examine what impacts breast shape in connection to pregnancy, plastic surgeons found that a history of breastfeeding, the number of children breastfed, the duration of each child’s breastfeeding, or the amount of weight gained during pregnancy were not significant predictors for losing breast shape. However, body mass index (BMI), the number of pregnancies, a larger pre-pregnancy bra size, smoking history, and age were significant risk factors for an increased degree of breast sagging. Nearly 104,000 women had breast lifts in 2006, up 96 percent since 2000, according to the ASPS. In addition, more than 329,000 women had breast augmentation, making it the top surgical cosmetic procedure in 2006.