I'd like to point out a few general things about mental illness and mental illness and the legal system, and I apologize in advance if this has already been discussed recently in this thread (I didn't see it if it was). First, because a person commits a crime it does not mean they are mentally ill. As for Casey, I personally have no idea what her mental health history is, or if she's mentally ill, as I'm not privy to the information that would be assessed to determine this.
Second, the majority of incarcerated individuals are not diagnosed with an Axis I serious mental health disorder, such as Schizophrenia, Bipolar Disorder, Schizoaffective Disorder or Delusional Disorder, for example. I work for a major metropolitan felony probation department supervising offenders who have been diagnosed with a serious mental illness (SMI). We have approximately 4000 felony probationers on active supervision department-wide; of those, about 400 are supervised through the Mental Health Unit. Having a serious mental illness does not mean a person is more likely to be violent per se. Some individuals with a mental illness do experience homicidal, assaultive or aggressive behaviors, but this is by far not the norm. I can't agree with the assertion that 99% of all persons in prison have a mental illness.
Third, personality disorders -- such as Antisocial, Borderline, Narcissistic, etc -- fall onto the Axis II diagnostic spectrum, not the Axis I. It is possible to have both an Axis I and an Axis II diagnosis, or just one or the other. Having an Axis II diagnosis alone does not qualify as person as "mentally ill." It just means they have a significant character defecit.
Fourth, it is entirely possible to be mentally ill and actively experiencing psychotic, manic or otherwise debilitating symptoms of a mental illness at the time of the commission of a crime, and still not qualify as legally insane. Not Guilty by Reason of Insanity is a very specific finding -- it is rarely a successful defense, as you all probably know. Andrea Yates? Totally, unquestionably mentally ill -- Not Guilty/Insanity was a very appropriate verdict in that case, IMO.
From what I've personally read/reviewed/watched in this case, I would not be overly surprised if Casey Anthony is found to have an Axis II diagnosis. I have no information about an Axis I diagnosis, though, and haven't come across any information that indicates Casey has an established history of Axis I mental illness. If anyone has read otherwise, please let me know (link plz'nthx!)