I think there is heredity, but not in the way that there usually is. In the normal manner of things, if a parent has inheritable disease X, then there's a chance that the child will get it, no matter what.
With us, though, BPD depends on a multitude of factors, many of which are the product of our environments. We tend to start off with a genetic disposition towards being highly emotional, but as children, we also learn things from our parents. If a parent has BPD and is responding to certain stimuli in a given way, then the child is more likely to learn to respond to it in the same manner.
So while it's still not considered a genetically inheritable disorder, the groundwork CAN be inherited from the parents and the rest of it learned during the formative years, almost making it inherited. The truth is, BPD or no, children learn a lot from their parents, good and bad, so unless they happened to be raised by Ward & June freakin' Cleaver, they're bound to pick up some less than perfect habit. Heck, even the Beav wasn't perfect!
Now before anyone starts a flame war, no, I'm not blaming my BPD on mommy and daddy. Funny enough, it was my maternal grandmother who showed all the symptoms, although she was diagnosed with Bipolar (though back in the 70's, when nobody was diagnosed BPD). Even before I knew my diagnosis, I knew everyone else was afraid of her temper except for me. She and I got along famously, for some strange reason. I guess, now I know why!