Interesting comment, John. I'll have to look that one up. However, lack of information has never stopped me from commenting before, and I don't want to stop now.
The first thing to note -- and maybe it's already obvious -- is that the Ohio decision does not "extend" Tarasoff. Tarasoff was a California state decision which has no legal standing in any other state (although lawyers and judges in other states may consider the logic that the California Supreme Court employed). Some states have affirmed some duty to warn or protect; a few have not; and many (such as Texas) have not acted one way or the other.
It is important to note that Tarasoff and other states' legal decisions do not create a duty for therapists when they find that there is a duty; but rather "find that a duty exists." This is analogous to Columbus's discovering America (insert Leif Erikson or your favorite Native American if you like): He didn't create America, he merely found something that was already there. That tells me that a duty to warn or protect probably exists even in states that have not had a precedent-setting legal case or passed a statute about it. (And you don't want to be the therapist/clinician who becomes the test case for "finding" the duty.) (Note that this concept -- which is coming from a non-lawyer, remember -- is limited to civil law. I believe that in order for an act to be illegal [i.e., "criminal"] in the U.S., its illegality has to be specified in the law [e.g., statute, Constitution].)
A word about the concept of "should have known." That's not unreasonable, in my view, and it certainly isn't new (in Tarasoff-like cases and elsewhere). While one may argue over whether it is reasonable to say "should have known" in a particular case, it doesn't seem fair to absolve an evaluating psychologist or psychiatrist, for example, of any responsibility just because he/she says he didn't know about some easily-available recent history of violence, or didn't ask whether or not a severely depressed young mother had obvious thoughts that might indicate a real danger to her infant. Similarly, any therapist who accepts a severely depressed young mother for evaluation and treatment, for example, _should know_ that post-partum violence is something to consider, and should either be prepared to do a competent job or refer the patient to someone else (for consultation, at least). Note that this applies also to non-psychiatric physicians, such as family doctors, who treat mental disorders and symptoms in their patients. If they decide to diagnose and/or treat, then they are held, in my opinion, to the same standard as a qualified mental health professional. (I've testified in a couple of lawsuits in such cases.)