As I understand it (I shoot a D3 and a D200), if the angle of view and aperture remains the same between the two cameras, the dof difference is negligible. If I'm taking a picture of a person filling the entire frame with her face, I'll need a longer lens or move towards the subject with the D3, and conversely, I would need a shorter lens or move away from the subject with the D300. Since the objective here is to frame her face, there will be a difference in focal length and/or distance from subject. If we are enforcing a constant distance to subject, as well as focal length, in addition to aperture, then dof differences will be apparent.
I'm doing a really poor job of explaining it. Bjorn does a far better job than I can ever dream of doing. He even has pictures to back it up.