Well I'll be! How's anyone supposed to find it there?
In FTM '06 it was in the Master Source dialog. Attaching it to an individual citation is much better (now that I know it's there!); after all, it's not the reliability of the source, per se, but its reliability viz. a given fact that's in question. My grandmother's hand-drawn family tree would be quite reliable for my mother's date of birth; not so much for thrice-great uncle on my grandfather's side.
One last question: is there a way to filter based on source quality? Maybe I'd like to hand off only my well-documented research.