I said something to a nursing mom once - we were at a children's museum and as I walked past, I said I liked her sling. Then I noticed that she was nursing and said something like "Oh, and you're nursing, that's great!" She gave me a funny look and I smiled and walked away.
Another time, I ended up getting into a conversation with a mom at a food court about getting my DS to eat veggies. She suggested getting him to dip them in ranch dressing and I said he usually doesn't get dairy, only my milk. She leaned forward and kind of whispered that she was still nursing her two-year-old. I said, "That's great!" She got this thrilled look on her face and gave me a big hug, then explained that nobody supported her continuing to nurse and she was keeping it a secret from a lot of people because she knew they'd hassle her. I was so happy to be supportive to someone who didn't have a lot of EBF support but was still doing it.