It's really weird lads. It's common in America AFAIK, but not so much here. A friend of mine is living there at the moment, and gets asked frequently for her number by random guys. I don't know any girl who would go for it here since it's very out of the blue, but it does make us feel good!
I was in a bookshop and afterwards, while crossing a nearby bridge, a guy fell into step with me and struck up a conversation about the book he'd seen me buy. He then asked me to go to the cinema with him that weekend! I declined, as a default 'stranger alert' reaction, but looking back I think I should have gone for it! If it happened tomorrow, I think I'd still say no though. It's just too unexpected!
I also had a guy trying 'The Game' out on me in the Jervis Centre before. He was trying to do the insult compliment thing they describe in the book and hook me in with some other techniques, but he just insulted me a bit too much and REALLY annoyed me so I told him to fcuk off and stormed off.