So, AMD released a little documentation years ago. Nothing since.
This is from memory but didn't AMD also explain back then that they can't give out much information because they have an agreement with Microsoft to provide a fully encrypted path from a Blu-Ray disc up to the HDMI input of a monitor, in Vista and later? Giving out programming information would violate this agreement. So, they sold out willingly or were blackmailed to do that. After all, Nvidia and AMD are really small players compared to Microsoft or Intel.
As far as NVidia is concerned, I'm very happy with my GTX 670. Linux support is there and works fine, at least for 2D desktop and video acceleration with VDPAU. Only 3D-ish thing I use in Linux is Google's MapsGL in Firefox. Main reason I chose this card is Windows games. Might play some Linux games in the future again, if Valve is serious about Steam on Linux.