Yup, but that's not so much a gun-control issue as it is a psycho-control issue, and it is a complicated matter.
First of all, someone has to be diagnosed as having a mental disorder. Then, someone in a position of authority has to recognize that this person is not simply someone with a minor mental illness (LOTS of us fall into that category, just most of us haven't been diagnosed as such), but someone who is bughouse nuts and a probable threat to others.
That isn't easy. Most mental illnesses have a range of severity; Person A may be mildly bipolar but harmless... Person B may be extremely bipolar with highly aggressive tendencies when he's off his meds. They both have the same illness, but to a different degree. I'd be opposed to simply banning everyone who was ever diagnosed with a mental illness from owning a gun... that would discourage people from getting counceling and take rights from people who are no threat to anyone. Probably 90% of people who have been diagnosed with some minor mental illness are no more threat to others than Joe Average.
You have to seperate out the real loons and psycho-killers from the folks who just have anxiety attacks or minor neurosis. This is a tricky business and might be a little subjective. Futhermore you have to insure communication and cooperation from various departments and services and individual shrinks with the court system, and there has to be due process before someone is stripped of a Constitutional right.
Then there's the issue of just how easy it is to get firearms from the criminal black market, even if you're banned from legally buying them. :shrug:
Hey, I'd like to keep real loonies from getting guns too.... I'm just saying it is a complicated issue, and even if everything goes perfectly there are no guarantees.