Don't ever, never, ever ask if she has a boyfriend! Maybe she does, maybe not - so what, who cares? Maybe she does, but wants to have a little fun - your prompting about a possible "borefriend" will put her off the mood, she'll move on to another guy and get what she came looking for anyway.
Or maybe she does, but is also pretty curious about you and wouldn't want the existence of a boyfriend scare you away. She might even be considering dumping him in favor of you.
If however she blurts it out without prompting, it still doesn't mean anything. Maybe she really thought she wanted to scare you away, but she really didn't know what she was doing. Take it humorously, be light hearted and give her another chance.
HB: Sorry, but I have a boyfriend.
You: (laughing) Hey, I just met you and you are already telling me about your problems?
One reason she might be saying she has a BF is that she really does have a BF, likes you, but doesn't want to feel guilty about cheating on her BF. This is classical chick logic at work here and is good, because it means you are on your way to a fark, even in spite of the BF.
Basically this happens because she thinks that if she tells you about the BF now, she can relax and let you you fark her anyways, since its not her fault now that she f'd you, because, she did tell you about the boyfriend, and you ignored it (ie. she had no choice in the matter).
HER: I have a boyfriend.
ME: That's nice, so anyway (ignore the topic and so will she).
Her: I have a boyfriend
You: Thats good - I'm not really looking for a girlfriend either.
Try those out.