Sure. But (a) the earliest signings (one-finger or otherwise) were only on the forehead (cf. Tertullian, Hippolytus, even St. Cyril of Jerusalem); and (b) there are early examples of multiple fingers. I don't think we can make much hay out of the antiquity of this or that current practice, given the ancient diversity and the obvious fact that no one does it only on the forehead any more.
Here are two standard quotes (taken from the Catholic Encyclopedia online):
"In all our travels and movements", says Tertullian (De cor. Mil., iii), "in all our coming in and going out, in putting of our shoes, at the bath, at the table, in lighting our candles, in lying down, in sitting down, whatever employment occupieth us, we mark our foreheads with the sign of the cross."
St. Cyril of Jerusalem in his "Catecheses" (xiii, 36) remarks: "let us then not be ashamed to confess the Crucified. Be the cross our seal, made with boldness by our fingers on our brow and in every thing; over the bread we eat and the cups we drink, in our comings and in goings; before our sleep, when we lie down and when we awake; when we are travelling, and when we are at rest".