Girls with Boyfriends - What to do?

So I met this really beautiful girl a few weeks ago; we've been chatting on and off for about a year and had started getting closer. Right before I summon up the courage to ask her out, she starts to warn me that she has a boyfriend.
WTF!!! Why didn't she say something earlier? Why is she telling me this now?
I'm sure a lot of guys have found themselves in this position; where the babe you want is in a committed relationship. How do you handle it? Do you try to breakup her other relationship? Or do you just be patient hoping that her eyes will open and she will pick you instead? Maybe your style is to keep wooing her, after all, she doesn't mind the attention *cue from beat99.9fm