A trawl around the net with regard to this was fairly inconclusive. Some posting the problem but no answers, so I used my D2X which has the same matrix metering system to confirm my thoughts.
The camera was faced with a scene that had areas of various intensities of light falling around the various A/F points, not too dissimilar in placement to the D200. Darkest was black and lightest white.
Camera on a tripod set to MATRIX metering and PROGRAM mode. By toggling the active A/F point and watching the calculated exposure it was quite obvious that although the camera is metering the whole scene, prominance is given to the subject covered by the active A/F point.
I have done a quick and dirty job of roughly overlaying A/F points from D200 and D300 on an image. Due to the density of the D300 focus points quite a number of different points could have been used to achieve focus, but depending on the selection, a range of different exposures would have resulted.
This only applies to MATRIX metering. I did not try focus lock and recompose, but suspect that would probably have given yet another result.
I do not have a D300 so cannot perform a similar check but I hope this helps.
Attatched image is an overlay of the D300 and D200 A/F points on a scene.
Don
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