How can anyone say asking her out would be alright if she has a boyfriend? Even if they aren't crazy in love, you can't really do that. You can hang out with her more, spend a lot of time with her and make it known that you're available and waiting, but to openly pursue a relationship with her while she's already in one is too far.
Do what I said. Pursue a friendship, and if she likes you as much as you like her, everything else will fall into place. There have been plenty of situations where me or my friends were in a situation with a girl who had a boyfriend where they eventually broke up and we swept them up.
I think a few people have got the wrong end of the stick. Not once did I say in the OP that I was going to ask her out...like I've said numerous times, I'm not a dike and, whilst Oof's correct that there really isn't anything to stop me, unfortunately my morals would. Especially if she were to agree to a date and come out with me while she was still with him...I wouldn't want that, let alone ask for it.
I was just going to let her know that I liked her (I've talked to her alot more now, and nothing's changed there) and that, whilst I'm cool with us being friends, I'd feel like I was lying to her if I didn't tell her the truth. Or something like that. Basically, there would be no date.
Bear in mind that I'm in the UK, where dating is completely different (the whole 'exclusive' thing is kind of implied over here)