Boobaa, on the actual street I tend to be more guarded, but I live in a big city where there are crazies roaming around. I've been accosted by crazy people randomly trying to come up and insult me or worse, so I tend to be pretty cautious when a stranger approaches.
But in a shop or other public place like a library, I would definitely talk to someone who didn't seem completely insane. In fact, recently I've been taking my laptop to the coffee shop so I can meet new people there. Last time I went a cute Aussie guy started talking to me. He asked me if my laptop was a gaming laptop and we started chatting. Unfortunately my cell phone rang, and I got totally cock-blocked (or the girl equivalent) and couldn't continue the convo with him.
I've also had situations where I'm sitting on a subway bench reading a book and a guy asks me what I'm reading. Unless he has crazy eyes or something, I'll usually answer and strike up a conversation.
Sure, public places aren't the best way to meet people, but not the worst either. I say go for it. You'll know pretty quickly by body language whether or not, you're barking up the wrong tree.
“Inside every cynical person, there is a disappointed idealist”--George Carlin