I've just stumbled on to this thread - and I agree with Alan. I've often overexposed on my D40 because I've been in aperture priority mode and forgotton about it. I've been on max aperture (e.g. f/3.5), taking a shot in the sunshine, and it's come out overexposed. My D40 warns me though, with a message something like "too bright" on the screen. You haven't had problems with your 18-200mm overexposing, but you have with the 70mm f/2.8! That makes sense too (if you're on aperture priority), because f/2.8 is much more likely to overexpose than the max aperture of f/3.5 on your 18-200mm. That's my simple response. I can't get any more technical or complex than that because I don't really understand how and why lenses might meter differently etc. Try looking out for warnings on your camera screen. If my D40 gives such warnings, I'm betting the D300 does does. Good luck!
