Hello. I haven't read through all the posts in detail, but I am writing as a clinical psychologist who's worked with a number of people with "BPD" and Bipolar. The first thing I want to tell anyone who's thinking they've got these issues is that they should probably be wary of anyone labeling them as "borderline". This is kind of a "garbage can" category for some groups of psychologists that means, essentially, that a female patient is difficult and particularly challenging to work with. It's like a buzzword. As a woman and feminist, I would not allow my colleagues or trainees to be so cavalier as to suggest giving anyone this label without serious debate. It's not really a diagnostic label that opens a path for helping or ameliorating symptoms. It is most probable that the the DSM-5, the next version of the diagnostic and statistical manual, will have many changes to the axis 2 (Personality Disorders) category. The fact that BPD is applied mostly to females sends up huge red flags for me. Also, most of the "symptoms" of BPD appear in many other diagnoses. In other words, it's very heterogeneous.
Bipolar Disorder, on the other hand, is a whole 'nother matter. As someone posted earlier, there are many gradations of the disorder, but the key feature is periods of hypomania or mania. In other words, along with periods of depression, the individual experiences periods of sleeplessness, agitation, racing thoughts. These periods can be extremely productive. Many highly intelligent and creative individuals have periods of hypomania, and indeed, many artists, writers, intellectuals, are thought to have been bipolar. On one end of the bipolar spectrum, the individual can identify a single hypomanic episode, while on the other, the individual has many or frequent episodes (rapid cycling). Also, however, some periods of mania may come with notions of grandeur or omnipotence. The depressive periods, on the other hand, come with severe self-loathing and ideas of self-anihilation.
What's important about correct diagnosis is that if somone is bipolar and prescribed antidepressants (as might be for someone who is "borderline"), the SSRI's can cause bipolar symptoms to be worse. On the other hand, mood stabilizers or antipsychotics (as given in some cases) would probably not be helpful to someone who has "borderline" symptoms.
So, I guess the grain of wisdom in all of this is: Do not accept a diagnosis of BPD without putting up a fight, because more and more clinicians are beginning to see that it isn't a helpful diagnosis. Also the "empirically-driven" therapies developed for BPD don't really get to the heart of the matter. Yes, it's important to give an individual who has highly destructive relational patterns an opportunity to develop better coping and relating skills, but most of these therapies stop short at getting to the heart of the matter: Most women with "BPD" symptoms come from very difficult family backgrounds where they learned their dysfunctional relational patterns. Therapies that don't also look at the foundation of the individual's personality are really nothing but band-aids in the long run.
Anyhow, I hope this is helpful.