I agree that the criteria shouldn't just be "mental health disorder." It can only be a factor if increased risk for violence can be measured basically independently of whether the person has an official DSM-IV condition. The Colorado shooter definitely would have fit-- it wasn't that he had a mental health issue, but that the psychiatrist specifically warned the school he might be a danger to others.
This is the correct response. Crazy people who have violent tendencies shouldn't be allowed to have guns. A broad brush of banning guns from anybody that's considered seeing a shrink would be wrong.