I have figured out why the brightness appears to be better on the 3G but is actually better on the 3GS. On the 3G the auto brightness feature did not work well at all in dark situations, only in normal to bright lighting conditions. The ambient light sensor on the 3GS is actually much more accurate especially in dark lighting than the one on the 3G ever was. So try turning the auto brightness off on both of the units, turn the brightness up half way on each, and then you will see that the 3GS is actually noticeably brighter now.
