I'm another double-diagnosis of bipolar with ADD. The ADD dx was easy - most of my immediate family has it too - but I think that delayed the bipolar dx because we always assumed my hypomanic symptoms were mostly ADD stuff. ADD and bipolar are two of the most inheritable mental conditions, and there are some similar neurochemical issues in both. For example, dopamine is a key player for both (a very complicated business, that particular chemical!) and norepinephrine can be implicated too - perhaps why Wellbutrin works well for a lot of bipolar people without the manic trigger effects, as it's a norepinephrine-dopamine reuptake inhibitor.

I do agree that it's overdiagnosed in young kids, who tend to have ADHD symptoms even under normal circumstances.