Ah! Just found this thread.
DUDE!! Go for it! The same exact thing just happened to me/is happening to me. Except this girl had been with her boyfriend for two years and I've been digging her for even longer than that (we've been best friends for 10 or so years).
I had enough this year and just before her prom I made my move. Totally made her dump her loser boyfriend, went to prom with her and spent prom night, morning-after, etc. with her.
Haven't looked back since and we're coming very close to seriously and "officialy" becoming a couple. Never felt better in my life, man.
Seriously dude, I would have regretted it so fucking much if I hadn't made my move and watched her go to her prom with that loser she was dating. Now that I've told her how I feel, she told me she's felt the same all these years for me but never had the guts to do anything about it. Best decision of my life.
Don't miss out on your opportunities. You don't get too many things handed to you in life, especially girls.