He likely filmed the role straight after The Good Shepherd, with Romanian reports of Matt's appearance in the film circulating in early February.
First confirmed details were from the Daily Mail on 19 May in an article about actor Tim Roth:
Still, he could be back in Cannes next year - not as a juror, but in Francis Ford Coppola's new film. It is Coppola's first picture in several years, an experimental drama called Youth Without Youth, based on a novella by Romanian philosopher Mircea Eliade.
The movie, about an elderly professor whose body is rejuvenated when he's struck by lightning, was shot in Romania and also features Bruno Ganz, plus an unbilled cameo by Matt Damon.
Coppola spent two million bucks of his own money to make it. "We shot with a small crew, but I have to keep schtum about everything else," said Tim.
Other sources have been a bit more forthcoming, though. I've heard that Roth's central character finds himself a fugitive, and also that the film spans from the late 19th century to the Sixties and is nowhere near the scale of the director's landmark Godfather films.
And that makes four films that Matt will appear in at the end of year (if indeed all are released, as scheduled, in 2006), with all four likely Oscar contenders: The Good Shepherd, The Departed, Margaret and Youth Without Youth. A nice bit of continuity, too, with Matt the star of Coppola's previous film, The Rainmaker (echoes of Good Will Hunting/Finding Forrester for Van Sant).