Three things:
1.: As far as I know, they
have released all the technical specifications, the radeonhd-driver being the first result, an open source driver that
should support all the features etc., but: This driver is hardly usable yet, currently, it does not even support 2D-accleration! However, being an open driver, it is very promising, it should be a great thing in a few months or maybe years.
2.: ATI employs two (again:
two) programmers for the Linux-section. nvidia actually employs about a
dozen or so.
This is the main reason why one should not use ATI in the near future and consequently the reason why that nvidia-driver is
ways better than anything from ATI. You are perfectly right with your saying about the new cards by the way, because if the radeonhd-driver mentioned above will support cards, this will very likely exclude older cards.
3. There is a certain reason why neither ATI nor nvidia like to release full specifications, because each of them cloned so many features from the other company, the moment one of them releases full specs it may get sued to the ground by the other company.
However, I am very happy with my nvidia-card now

Not because it has better results (I think the ATI-card actually may have worked better), but because it
works.