I've also read that one of the major differences is that it is a Personality Disorder, whereas BiPolar is a Mood Disorder. Also, BPD is dependant on a situation and as situations change we change too (we are more prone to being able to flip from one mood to another via listening to different music, reacting to someone's comment etc) whereas BiPolar does not require a situation to trigger it, the moods change and swing not really in relation to an immediate trigger, but to an underlying chemistry that sort of swings at will. It itsn't something that can be 'snapped out of' whereas someone with BPD can actually come out of that mood when something really positive suddenly occurs. Hence the 'roller coaster'.