KrazyKat, you make a lot good points. What is so interesting about mental illnesses is that there are genetic components, but not so much one gene (ie. the bipolar gene). (exception to diseases like huntington’s disease which is more a neurological disorder). Thus different people who satisfy the bipolar diagnosis have a different combination of genes involved, not even thinking about the environmental factors.
Many developmentally disabled individuals demonstrate many symptoms of ADHD. These individuals also have executive deficits, probably mostly by different mechanisms. Many are treated with stimulant medications. I’ve seen stimulants used in Traumatic brain injuries, autism, mental retardation, fetal alcohol, etc. One cannot really make the argument that the fact that one uses the same class of medication to treat ADHD makes it a single disorder. Anticonvulsants treat epilepsy, bipolar disorder, chronic pain, migraine headaches, and aggression.
Versus describing different disorders, perhaps, ADHD is more like a spectrum disorder? Life is certainly not as black/white as the DSM-IV! I think the DSM V is going to change bipolar disorder and autism into spectrum disorders. The mental health professionals continue to argue with each other on these distinctions. very interesting.
yay for stirring up those brain juices 
you are awesome