Nvidia began porting PhysX to its GeForce cards shortly after acquiring Ageia and is due to ship drivers enabling the new feature soon. While there's no official way to run PhysX on AMD hardware, in theory it's entirely possible given that all the materials needed are publicly available.
Well, Eran Badit of NGOHQ.com has already made some considerable progress getting PhysX to run on a Radeon HD 3870 card and it looks like he's now getting help from a rather unexpected source. He has not only been invited by Nvidia to join their developer program, but has also been offered SDKs, documentation and hands-on help from actual Nvidia engineers.
Interestingly, AMD is not thrilled about this and has been unwilling to provide with any hardware or support for the project - possibly due to its backing of Intel's Havok physics engine. With or without their blessing, however, a successful port might be just around the corner.