Answer:
Yes ADD is a disability that is in fact recognized and included in the definition of disability under the Americans with Disabilities Act.
That said however please remember that ADD is also often something that can be accomidated for in social, educational, and work settings. Once someone is properly diagnosed and assessed, there are a varity of accomidations (many of which are amazingly simply and cheap) that allow the person to manage their ADD symptoms.
Generally speaking ADD is considered a learning disorder. Statistically however as a group, persons with ADD have on average a higher IQ than the population as a whole. One reason this fact is possible is that ADD has little to do with a persons cognative capacity (or inate inteligence) and more specifically everything to do with the persons ability for cognative procesing. In short the problem for a person with ADD is not inteligence, but more about focus and productively dealing with stimuli.