I've been getting it more and more, since about 21 weeks. Before that I did have some random comments, as well as tons of people who would stare and look like they wanted to ask but didn't.
A few people have told me they can't necessarily tell just from the size of my belly, but that it's obvious from my posture when I walk. Which is probably not good news for my back, but at least it keeps people from thinking I just hit the donuts and ice cream too hard!