Computers are a cutthroat business, and often times compatibility has nothing to do with the technology at hand, but licensing agreements and corporate politics. All that stands between SLI on an Intel motherboard (or CrossFireX on Nvidia silicon) are drivers and a BIOS tweak. Don't believe it? Look at HP's Blackbird 002.
Now take that same concept and apply it to the heated GPU wars. With AMD partnering up with Havok and Nvidia acquiring AGEIA, the race is on to not only deliver the fastest graphics card, but physics acceleration too. Of course, developers would prefer one standard, and Nvidia indicated it would make PhysX available for free through its CUDA SDK, but if ATI had any plans of going that route, it appears they've already been beaten to the punch.
Consider it a proof of concept or a smack in the faces of both ATI and Nvidia. Either way, Eran Badit over at NGOHG.com took matters into his own hands and developed a driver enabling PhysX support on his HD 3850 videocard, whether the two companies approve or not. According to his screenshot, the modified driver netted an impressive 22,606 CPU score in 3DMark Vantage, and an overall score of P4262. Eran initially indicated he would offer the drivers to the public after more testing, but has since said "the HD 4800 family won't be supported, because AMD thinks NGOHQ.com is not worthy enough to borrow review samples (definitely a bad idea). Anyway, expect to see some cool video demo video in several days-weeks." In other words, stay tuned.