BPD and a lot of other conditions have similarities, at least superficially. The motivations behind these similarities differ.

BPD and ADD similarities: irritability, affect lability, stress intolerance, poor self image, a tenuous identity, lack of direction, disorganization, impulsivity, substance abuse, low self esteem and interpersonal problems.

Chronic feelings of emptiness, fear of abandonment, chronic emotional crises and suicidal behaviors seems to differentiate ADD/ADHD from BPD, if there is no comorbidity. These features are present in "normal" people as well (from time to time), so I think it has to do with how much those features interfere with day to day living and relationships.

Another major clinical difference is the ability of a person with ADD/ADHD to form a stable relationship with their therapist.

Here is a good paper that explains the differences: http://www.springerlink.com/content/r7q47qw007h87431/