I've played RPGs in public. This being New York City, most people around are completely disinterested and leave you be, although you do get a few curious onlookers. Their reactions are generally positive, even if the "that's cool" responses far outweigh the "how can I learn more" ones. Most people seem to get it that continuous questions would be distracting to the game and keep them to a minimum. Haven't had anyone ask to join in yet though.
I'd say we are more restrained with the volumes of our voices in public than in a private home, mainly because we don't want to disturb others enjoying the space.
Most of my friends that I play with (myself included I hope) don't fit the "typical geek/nerd" stereotype, so I don't worry too much about that. Then again, when you work on Wall Street and you're playing a casual lunch time game in Battery Park in a $2,000 suit, it's kind of hard to look like a loser.