Bipolar disorder is essentially a mood fluctuation disorder, characterized most often by depression. Bipolar people can abuse substances, but otherwise they do not typically show the range of different symptoms BPD people manifest.

BPD individuals can be depressed, but BPD is essentially a hyperreactive/impulsiveness disorder with mood features, but also other symptoms affecting thinking and perception, self-concept, relationships, etc.

BPD involves instability not only of mood, but of perception of self, perception of others, relationships, behavioral consistency, self-control, etc.

The mood disturbances in Bipolar Disorder typically are along a depression--mania dimension; (mania, hypomania) with BPD, however, mood disturbances typically involve the dimensions of anger and anxiety.

Most BPD patients do not have mania (sustained hyperactivity and elation), though they can often act impulsively." (Daniel C. Claiborn Atlanta)

From: http://www.aapel.org/bdp/BLbpd-bipolarUS.html