If you like conspiracy theories, then this one shoud be a no brainer - Arstechnica is reporting that NVIDIA may have deliberately crippled PhysX performance on the CPU by making it use the older x87 floating point instructions which have been replaced by the newer and faster SSE instructions since 2005. NVIDIA is obviously denying this is the case, and blames an old codebase for the problems. They have however, stated that the next version of PhysX, versions 3.0, will resolve these issues. We'll have to wait and see.
arstechnica.com - A new investigation by David Kanter at Realworldtech adds to the pile of circumstantial evidence that NVIDIA has apparently crippled the performance of CPUs on its popular, cross-platform physics acceleration library, PhysX. If it's true that PhysX has been hobbled on x86 CPUs, then this move is part of a larger campaign to make the CPU—and Intel in specific—look weak and outdated. The PhysX story is important, because in contrast to the usual sniping over conference papers and marketing claims, the PhysX issue could affect real users.
Full article here.