Bipolar Disorder. What are we really talking about?

As a clinical psychiatrist, I have a lot of thoughts about studies, genetic or otherwise, of the various mental disorders.  Before we even look at the merits of a genetic association study for a particular mental disorder, I think it's worth looking at a few diagnoses and some of the possible pitfalls of doing a study specific to a diagnosis.  Some diagnoses might have a more "organic" flavor to them (Schizophrenia, Schizoaffective Disorder, Bipolar Disorder) and one might assume that they would lend themselves better to genetic association study.  So I'll start with one of those: Bipolar Disorder.