http://w3.uchastings.edu/plri/spring95/tmpinsan.html
Interestingly, this page notes that if the defendant does not initially plead not guilty
by reason of insanity, then the court does not have to accept an insanity plea later on. The defendant in this case has, thus far, pleaded only "not guilty". Although in this case I believe they are considering amending or changing the charges in light of this new theory, so that might change things.