The new physx egine for th Nvidia card (CUDA) was leaked a while ago, since then a website have been changing this driver to become compatable with ATI GPU's (including the 4800's).
http://www.ngohq.com/news/14254-physx-gpu-acceleration-radeon-update.html
is the link to the website of the developer and the shocking truth that nivida is helping him with this project
It has also been suggested that due to nvidia'd help he has been restricted to releasing it only after when nvidia has realeased their public version of the driver