Decaf would probably be the best one to explain this to you...
Oh jeez... pressure
I haven't read anything I'd trust that deals with how MBTI and most psychological dysfunctions interact, but I have a theory (run away! save yourselves!). There's a part of our brains that is highly active when we are sad or depressed and dormant when we're happy. Having dealt with a number of depressives I started reading up on new neuroscience research where they are developing a first generation brain pacemaker that "settles" that area of the brain when it becomes overactive (because the brain lacks the enzymes that shut down the "sad center" of the brain when the brain would normally trigger that reaction). OK, so that doesn't really deal with bipolar, but my theory is that bipolar is the brain attempting to be its own pacemaker, calming down the overactive depressive center, but it overdoes it. The manic side of bipolar is due to the completely dormant center that should normally have activity, and the brain shutting down the calming effect, which lets the center go back to overdrive. Basically, I believe bipolar is a disorder of that function that regulates the corrective part of the depressive center.
*gasp* *inhale*
MBTI, right. Your four letter code helps define what order your functions are in. So for INTPs it goes:
Introverted Thinking
Extraverted Intuition
Sensing
(There is still disagreement on whether or not this is extraverted or introverted)
Extraverted Feeling
When we are in a good mood, we find that we are confidant in our top function and enjoy the creative use of our second function. That's how we are most productive and tend to be most skilled.
When we are sad, however, we lose confidance in our top function and lose our desire to be creative. We end up relying on our least developed functions and our table flips upsidedown. We end up using immature versions of our third and fourth functions instead. For INTPs that can exhibit itself as attempting to blame the world and its misplaced values (extraverted feeling) for our problems. The way that most INTPs that I've worked with deal with this is trying to engage their slightly less immature function (for me its extraverted sensing) to rebuild confidance in myself. I proofread old work, or I read a reference book I'd been meaning to get to. Its very tough to learn as a habit, but if you can harness your down-state, you may find that you can be productive and feel better about it later (because obviously you can't feel better about it when your depressive center is still in overdrive).
I believe bipolar would make it hard to type you, but not impossible, and I don't think that it pushes you into behaving like another type... just a depressed version of the type you already are.
Oh, and yes I agree that personality type is only a facet of what makes up a person's mental process. We define what we can in an attempt to find clarity, but people are just too complex for a system of only 16 types. Maybe if we had a system of 5 billion types we'd stand a chance.
*edit* actually, now that I think about it... the only reason my apartment is clean is because sometimes I feel frustrated and sad. I can' motivate myself to do much of any real cleaning if I feel good about myself. Go figure.