Well it's not a jack off scene but its a nice scene. The movie "Shadowboxer" (2005) has a nice scene with Stephen Dorff fucking a girl when he gets frustrated from talking in the next room he pulls out of her completely nude and grabs a gun and walks off. Mind you can see his whole semi erection and promoting condom use.
An 80's movie called "Spetters" has three guys in a garage comparing/measuring thier members and finishing in a circle jerk- full frontal.
I rented "The Nines" yesterday and Ryan Reynolds has a masturbation scene in it. It's kind of a strange movie, and Ryan doesn't show much skin, but if you're a Ryan fan it might be ok.
Christian Schaech (the ex of Christina Applegate) does a voyeuristic wank in one of those Greg Akari movies (cannot remember the name now.). When he is done, he happily devours his cum.