Well, I'm also very excited about hardware PPU. But I'm old enough to have been one of the first adapters of the first 3DFx add-on GPU. As soon as the major graphics card producers got into that area, 3DFx quickly became obsolete and incompatable. Perhaps if they maintain DirectX compatability, this won't happen quite so fast with the PhysX chip, but I'll bet these first add-on PPU cards will be relatively short-lived.
My guess is that within a couple of years, no one will be writing games for the PhysX and we'll all be using Nvidia or ATI graphic cards with integrated PPU's.
However, I'll still buy an AGEIA card as soon as the apps are there to support it. But I'll do so, knowing that its probably a short-term solution.