Originally Posted by Ninja Prime
It wasn't due to undersampling, or rather it was due to undersampling, on NVs side. The fix was to do the same undersampling NV was doing, bluring the lines between mip map layers. ATI was using the proper directx defined method, with a sharper texture, but the sharper texture was causing the shimmer. You can try this yourself but cranking up the quality on NV cards to max and it will get the same shimmer. There was a huge thread at B3D about it.
I'd still like a link to something that says it is fixed beyond some users on another forum, that just happens to have been owned by a guy that ended up working for ATi PR.
AFAIK this hasn't been fixed, and ATi cards still shimmer.