Last night, there was a fascinating NOVA show on PBS. Experts were investigating whether a portrait of a girl on vellum (shown above) is really a Leonardo Da Vinci. They think it was cut from an old book of the period (they found the book in a museum in Poland).
Link to article about the story in the Guardian.
Here's the link to the show which you can watch online if you missed it.
I blogged about this picture earlier and said I thought it looked to me like it might be authentic. That was just my gut feeling based on the style of the face and execution of the drawing.