Yes, birth control can prevent the egg from attaching itself to the lining of the uterus, but I would call such an incident prevention before i'd call it k/illing - and as eiri said, it's very rare when this happens. It's not as if a woman would know she's got a fertilized egg floating around in her uterus.
Also, hormonal birth control is most definitely not a form of abortion - if it was, there would be no need for abortion clinics. Women would just down a packet of pills when they are suspicious of pregnancy if the pills could be used for this purpose. Along the same lines, I wouldn't call plan b an "abortion pill" either - it's purpose is the same as birth control pills: prevention.
Interesting topic. I'm kind of eager to see what kind of bickering this topic elicits.