I had IVF and 7 days after my transfer I started bleeding very very lightly (literally a few big drops). I was certain that it was my period, even though I wouldn't normally expect it for another 7 days, and already counted this cycle as negative :( However, the bleeding stopped that afternoon and the next day my preg. test showed a definite positive. I also thought that implantation bleeding was some kind of a myth, but what I had kind of matches that definition.
I am 17 weeks pregnant now, and that was the only time that I bled since then. I agree with Annie, though, esp. if it happened on the day of your expected period, it is most likely your period...
Implantation bleeding is much more talked about and hoped for than seen. Women who don't want to believe the evidence of a period will often leap on some irregularity about the period and decide it was really implantation bleeding, (and then explain the subsequent negatives on pregnancy tests as themself being one of those ladies for whom pregnancy tests don't work).
I'm not saying implantation bleeding never happens, but we don't hear comments on this site from women who write and say they in fact had it. An embryo implanting is no bigger than the dot on the letter i here, and the amount of blood it might release in burrowing into the uterine lining will be negligible. Good luck, I hope you prove my comments wrong.