Over at Legal Ramblings, Steven Wu discusses "guilty but insane" as proposed over at NRO.
At a theoretical level I agree with his comments but I must point out that this sort of finding is already available in some State legal systems.
Personally, the only way I think a "guilty but insane" finding might be correct is if it is a sort of reverse "voluntary intoxication" theory. If it can be proven that you were previously diagnosed and treated for whatever condition but did not continue treatment (something that happens with a number of my clients) then you could be held responsible for your actions. But other than that I just can't see it.