Looking at Mike's comparison with FSAA...I noticed a few things:
1. ATi's FSAA seems to do more work overall. I noticed with nVidia's FSAA it seems to do some sort of distortion or blur trick. Whatever it is, it seriously affects the form of the central area of that object (both in colour and geometry).
2. Some of the aforementioned distortion still yields a nice effect on some of the edges ATi's FSAA doesn't quite hit as much.
3. 6XS and 8X for nVidia look surprisingly similar except for 6XS causing an unusually high level of blur and distortion.
Overall, I got the impression that nVidia's FSAA blurred and distorted the edges in order to remove extra aliasing. The end result is some of the spikes changed in width and colour (and some other slight geometrical variations).