Man, I know how you feel. It seems like whenever I'm even remotely interested in a girl and become friends with her, I then find out she's in a relationship already. It kinda bums me out since I think I've finally connected with someone then to find out they're taken. I still remain friends with them, but it's also feels kinda wierd for awhile.
Thank god I'm still in college and can use tools like thefacebook.com to do some preliminary research before making a move. It's saved my ass from awkwardness before
-Robert