Is there a consensus as to whether Atkins requires that there be a separate jury trial as to mental retardation prior to the actual death penalty trial OR is it sufficient to have it as a third special issue during the punishment phase?

Look at Ex parte Briseno, 135 S.W.3d 1 (Tex.Crim.App. 02/11/2004). The Court's reasoning is instructive.

There's not even a consensus that it has to be determined by a jury. See Ex Parte Briseno, Ex Parte Hall and Ex Parte Simpson (all acknowledging findings of fact entered by the trial judge on the M.R. issue). I'm not personally aware of any instance where there's been a separate jury trial on the mental retardation issue in a death penalty case. I do think there are several counties (ours included) who have prepared an additional special issue on retardation to be included in the punishment phase jury instruction. Briseno stands for the proposition that the facts of the crime are relevant to the M.R. issue so there's really no good reason to have a separate trial. You'll just wind up trying your capital case twice. Also, if you do give the issue to the jury by way of special issue, it might not be a bad idea to request findings from the judge as well. You'll probably be better off in the long run.