Bipolar disorder and autism spectrum disorders

Do you think it is common for a young person with the emotional volatility associated with Asperger's (or, as it is now included in the more general, Autistic Spectrum Disorder) to be incorrectly diagnosed with Bipolar disorder?

Yes, we know that happens quite a bit in practice, although we don't know how often. Young people with autism spectrum often get quite angry when routines are changed - they expect things to go according to a predictable pattern, and when they don't, they become enraged. These "meltdowns" can last for hours and in some quarters get mistaken for brief manic episodes. Manic episodes should not be diagnosed unless there are the other signs of mania along with the anger and irritability: decreased need for sleep, increased activity, pressure of speech, grandiose thinking, impulsiveness, flight of ideas, plus irritability that is fairly constant over several days.

Also, one would want to rule out "disruptive mood dysregulation disorder" if it is a kid who has explosive outbursts on top of an ongoing irritable mood, without the other symptoms of mania or autism spectrum disorder.

Before concluding that the kid has autism spectrum, there should be the usual signs of autism in addition to the angry outbursts: trouble understanding one's own or others' feelings, repetitive behaviors, lack of social reciprocity, etc. In general, ASD has a younger age at onset (age 3 or earlier) than bipolar disorder.