A few times I've just flat out told them something and trusted them not to metagame with the information and I can't remember being disappointed in the few times I've done that.
Not directly related to the feature request at hand, but I just want to point out that not all instances of keeping information secret are due to not trusting the other players with metagame information. Most of the games I've played don't keep any secrets, but one game in particular made considerable use of short, private GM+player conversations. In those instances, all the players were on board that they, as players, would know only what their characters knew, and because of that, the added tension of knowing that another character knew something else, but not knowing what, added much to the general atmosphere of the game.