Several years ago, when my nephew was an infant, I attended a Pampered Chef party at my sister's house. She also invited some of her friends who had children the same age as her son. At one point while we were passing around various products, I turned to hand one to the lady sitting next to me, only to discover that she was nursing her son. It startled me, but I didn't say anything or stare or anything rude. She was mostly covered by her shirt and the baby covered most of the rest. It had actually taken me a few seconds to realize what I was seeing.
I did mention it afterward to my sister, saying that it had basically startled me. Her response was that it was perfectly natural and she wasn't flashing anybody. It was more an embarassment on my part then anything the woman had done. I'm not even sure anyone else had noticed.
If he baby is hungry, feed it. Breastfeeding is natural and it's great that most women choose to feed their babies that way. There are too many women out there that flash their bodies just to get a reaction and it's sad that people have to jump all over someone who is just trying to feed their child.