hehe I just happend to read about it two minutes ago, I'll try to explain.

Ah, now I understand... Here it goes... (example)

Render Globals;
--> Resolution --> Preset = 640x480
--> Render Options = uncheck "ignore film gate"

Camera attribute editor;
--> Film Back --> Film Gate = 70mm
--> Display Options = check resolution gate and film gate

And there you go!

Another thing; in Camera attribute editor -->
Film Back --> Film Fit = Horizontal or Overscan to get the film gate inside your resolution gate.