ok just throwing my 2 cents in here.
just because someone has a bf/gf doesnt mean they arent available. LOTS of people are in relationships that suck. why do they stay? because they are afraid of being alone. they become coodependent on the other person, even when that other person isn't good for them.
so follow a simple rule in life... try honesty!
instead of all the games and over analyzing, just tell people how you feel.
this guy WAS flirting with you, and the reason he was was because his gf isn't the one he wants to settle down with.
now here is the tricky part. people are cowards. so what they do is they flirt while telling themselves 'oh this is harmless. im just being friendly. nothing more' all the whil subconsciously hoping the other person forces their hand.
had you said to this guy. "Frank, i don't know how you feel, but I find myself attracted to you. i know you have a relationship, so i think we should probably cut back on how much we pal around."
now i can tell you right away he would have said 'ummm ok". and he probably would have pulled back from you. BUT what you've done with your actions is taken whats going on subconsciously in him and brought it to the surface.
now he starts to question himself about why he was flirting with you, why he's with a gf that isn't satisfying, etc.
now that he knows he has another option to explore, it would only have been a matter of time until he dumped the gf.
REMEMBER all relationships go through fights. and the next fight he and his gf had he'd be thinking to himself 'why the hell am i putting up with her crap when a hot little number appreciates what i have to offer!" at which point he dumps her and shortly there after asks you out.
anyway. you can never go wrong in life by being honest about the way you feel.
you do however have to have the strength to do it. it will kill the flirtation game going on right now. but thats because its a GAME and once you are honest you're upping the ante and making it REAL.
you could have easily had this guy. all you had to do was let him know you liked him and then waited. him and his gf eventually would have had a fight and he would have dumped her ass.
still not to late i suppose. if you have his email address you should email him and tell him that you miss his presence in the office and that if he's ever single you'd like to have dinner.
then forget about him and let nature take its course.
just my 2 cents.
J