The ipod/headphone thing is a complete limiting belief. I used to think, "Hmm girls wearing headphones don't WANT to be disturbed. That's their signal to tell us to stay away."
Haha, it was just paranoia and excuse making! This is all about value. Look at it this way, if you're walking down the street with headphones in, and some panhandler asks you for money, you probably won't respond. But if someone offers value to you, e.g. somebody stops you to offer you a Ł10 note, you will take out your headphones and listen.
Of course, we're not going to be offering girls money, but we offer value from our interesting personality, ability to have fun, make them laugh etc.
I stop girls all the time on the street wearing headphones, but I use Direct openers, which will get their attention quickly.
On the bus it's a little different, you're right to note that people will tend to be a bit more closed off because of others around. However, then it's down to your ability to make a situation light hearted and fun in order to relax her.
The key is making her laugh or giggle within the first 30 seconds of the interaction, if you can do that she will instantly relax. Use a slightly teasing line, e.g. tease her for a big bag and ask if she's got a gun in it, or if she's reading Heat magazine ask her what the latest on Britney Spears is and tell her you used to be a big fan ("They just don't make music like 'Hit me baby one more time' any more do they?").
All with a big smile and cheeky demeanour.
Soul