I have determined that if a stranger tries to touch my belly, I will scream. Plenty of people have touched without asking, though - nurses at the clinic where I work, for example, and random people at church.

I have a couple of friends who love patting my belly but are very respectful about asking every single time. They are wonderful. The only people who have carte blanche to touch without asking are my partners. Okay, and my midwives - although they usually ask, or at least warn.