PhysX is no doubt a cool technology and as we see more and more games make use of it, why wouldn't you want to use it? - The problem at the moment for NVIDIA is that no one with a high end ATI card is going to move over to a high end NVIDIA one. At the same time, if people want the fastest single card on the market, you can't look past the HD 5970.
PhysX for most people isn't going to be a big enough swaying point to pick one brand over another. Yes, the list of games that support the technology is big, but once you cut that list down to games you want to play and then cut that list down to ones that actually play better with PhysX, you're not going to have a large list of titles in front of you.
On the other hand, if you're using a HD 5800 or HD 5900 series card and for a few hundred dollars could add PhysX ability, the option becomes quite attractive. I could easily see people picking up a GTX 465 to go alongside a HD 5800 series like we have here today.
The good news is that these beta drivers aren't going to be hard to get a hold of and since they're part of the latest series and driver updates for NVIDIA aren't very regular, buying a cheap second hand GTX 200 series card or using one you may have lying around could be a good option for experiencing PhysX.
A GTX 465 over a GTX 260, while adding more performance, doesn't really add loads, so it's probably not warranted picking one up while the technology isn't officially going to be supported by NVIDIA.
In saying that, though, if NVIDIA did decide to support the technology long term, like I personally think they should, then I would suggest getting something like the GTX 465 which is going to handle more intensive PhysX better and if Mafia II is anything too go by, it's the perfect reason or excuse to nab an NV card to go along side your ATI one.
We have to wonder if the inclusion of PhysX support with an ATI card present was a mistake or if NVIDIA did it on purpose to see what kind of attention it would get. If it was done on purpose, then hello NVIDIA, offer it already!