The performance drop concerning ATI is not yet confirmed to be cheating until it can be shown that it in some way reduces image quality or does not fulfill the precise instructions sent to the driver by 3DMark2003, as was shown with the Nvidia drivers.
It may be that ATI has a 'game-specific' optimization in the mother nature test that does not take any artificial short cuts which reduce work load. THIS IS VERY IMPORTANT. If the ATI drivers are still doing EVERYTHING that the 3DMark2003 application asks, then it is not cheating.
If this is the case for ATI, I hope that they come out and say it so that they can dispel any doubts.