There are none so blind as those who will not see.
Forbin, do pay attention, I will say this only once more. Either you get it or you don't and if you don't get it by the end of this post, you never will.
First, to refresh your memory about texture smearing look at these two screenshots again:
ATI:
http://members.shaw.ca/sjdicks ... 960x32_4x8x_highlight.jpg
nVidia:
http://members.shaw.ca/sjdicks ... 960x32_4x8x_highlight.jpg
Does this look to you like ATI and nVidia are "both doing the same thing"? Does one of those screenshots look more "wrong" to you than another?
With those images fresh in your mind, let's move on.
With ATI, there is no need to use a negative mipmap LOD bias since ATI's anisotropic filtering does not produce texture smearing artefacts (see the screenshot above). Read that sentence over again until you completely understand it.
nVidia's anisotropic filtering does produce texture smearing artefacts (see the screenshot above) so you are forced to either live with the smearing or use a negative mipmap LOD bias and put up with the resulting texture shimmering.
You said yourself that you see texture smearing on your nVidia card when the bias is set to 'Clamp', right? You chose to use a negative LOD bias setting instead which means that you end up with some texture shimmering. Well, guess what? If you had an ATI card, you wouldn't see any texture smearing in the first place so you wouldn't have to use a negative LOD bias and therefore wouldn't get any texture shimmering either.
It's quite simple really: nVidia forces you into a compromise. ATI doesn't.
And on that note, I'm done. Enjoy your nVidia card.