Version: 2.69 official (win 64)
How to reproduce: Render First Scene. The orange cube is in focus, as it is being used as the active camera's focal point. The defocus node is inside a node group used by both scenes. Now, render the second scene. Notice that the orange cube is out of focus even though the active camera is also using it as a focal point.
Expected behavior: Since both scenes are identical except for the cameras, and both cameras use the same focus, the orange cube should be in focus for both scenes. Instead, it seems that the defocus node persists in using the camera distance data from the first scene in which it was originally added.