There are therapist who think that BPD it's not really a diagnosis, so that's why is easily mistaken for Bi-Polar Disorder. That's too bad because, even though the medication treatment is the same or very similar, IT'S NOT THE SAME PROBLEM!
Borderlines have drastic mood change in short periods of time, going from depression to mania to fury in very little time (sometime hours or minutes) while for bipolar those mood swings may change drasticly, but they last longer (months or years).
I'm talking from the experience I've had. I may be mistaken though.