Bit late replying, but I am new to the forum...
DowntownDC, I believe the reason that some people assert that the link with abuse is unclear is because of the high potential for parents with borderline personality disorder to be a source of extreme stress in their children's lives- in other words, it is possible that the abuse and abandonment suffered by those who grow up to have BPD have that abuse or abandonment inflicted on them by a parent who also has BPD. Thus, it is pretty much impossible to know if it is the abuse or abandonment itself or a genetic link from the parents, or more likely both. Personally I do not find "unclear" to mean "untrue." It simply means there is not enough evidence, which I can see in this case as being true.
Also, as Normal? somewhat pointed out, the diathesis-stress model (the concept I learned about, which sounds like it is the same thing as the Epigenetics that Normal? mentioned) is more simple than what you had thought- that the lack of abuse would have led to a different disorder. Actually, in the diathesis-stress model, you have a predisposition (genetic/nature component) and coming into contact with a stessor (ie abuse) causes the disorder to be triggered. But the lack of the stress, or a strong ability to cope with stress, the switch stays off which means the person does not end up with any disorder. The best example I have of this is Alzheimer's: my grandmother had this disease, while her twin sister did not. Her twin sister didn't have any sort of dementia at all, in fact. So there must have been something that triggered the Alzheimer's in my grandmother, but that her sister didn't experience.
~K