Same boat here, slightly different creek.
I've been friends with the girl for about two years, and for the past year or so we've gotten together to hang out just about every Friday night. I pick her up from work, we have dinner at some point (sometimes I cook, sometimes we get fast food, occasionally we'll go someplace a bit nicer), and we usually hang out at my apartment until after midnight. Pretty much a dating-style scenario by most standards.
At one point during the summer, though, she informed me that she had a boyfriend. The twist here is that she'd never actually met the guy in person; he lived all the way across the country and they'd met online. This raised all kinds of red flags for me, but I figured I had to let her make her own decisions. About a month later, she flew to the East coast to meet him and bring him back. Currently, they're living in the same house, if not the same room.
She and I still do our usual Friday-night thing, and he joins us every once in a while. Now I'll grant that I'm heavily biased?and I don't know what they get up to when I'm not around?but it still seems that she and I act more like a couple than she does with her "boyfriend." And, talking to some mutual friends, I've found that opinions range from not liking him at all and thinking he's creepy to wondering just why they're together in the first place (since it seems the only people who have any idea are the two involved) to being surprised that the relationship has lasted this long.
I'm still into her, and it feels like she's at least a little bit interested in me. (If not, she's doing a very good job of bluffing.) Her current relationship seems doomed to fail, and I feel like I'd have a decent shot with her when it does. But the waiting?and trying to piece the puzzle together for myself?is killing me.