Originally Posted by B-Dub
Diacetel is usually not detected until after bottling or kegging. I have never tasted diacetel in a fermenting or recently finished beer. It develops over time that is why it is a good idea to do a warm rest and not have to worry about it.
I believe this is true for lager strains, but not necessarily for ale yeasts which are fermented at higher temperatures. The precursor to diacetyl (acetolactate) is produced very early on in fermentation, and the rate at which it oxidizes to diacetyl in the fermenting beer is related to a number of factors that vary with temperature.
However, you are correct that failure to detect diacetyl at the end of fermentation is no guarantee that the finished beer will be free of diacetyl. I retract my earlier piece of advice -- it was flawed, as you were careful to point out, B-Dub.
Regardless, Cal Ale is well known to ferment fine without off-flavours at surprisingly cool temperatures, so I would be very surprised that a fermentation at 64 F would lead to excessive diacetyl production. But as B-Dub says, a quick rest at an elevated temp. is cheap insurance if you are still concerned.