To whit, they are releasing specs and some example code under NDA and an opensource library that connects to the card's BIOS. They already have XOrg developers onboard and have also attracted Jerome Glisse who reverse engineered ATI cards to make the Avivio driver (incidentally probably killing that driver but the new ATI open driver will surely benefit from his excellent work).
While not completely open, this is pretty open for starters, HOWEVER I for one am not certain of things like support for hardware video decoding or extra on board hardware (TV-Out etc.).
Checkout the final comments: "The aim of this open-source driver is not to overtake the fglrx driver but rather is designed for those who just want a working desktop with 3D capabilities and basic video playback. This new driver is ideal for FOSS enthusiasts and those wishing to run the latest development kernels and versions of X.Org."
Still there is no argument that this is a brilliant result.