Sort of. Like Havok you have have a PhysX game that will do the physics on the CPU. Or you can have a GPU PhysX game that runs SOME things on the GPU. Batman:AA is one such game. They can't require an Nvidia card or they'd limit their audience to bad. You can turn PhysX off, or have it on a few different settings. By turning the PhysX on, you are simply adding detail. I haven't seen any reviews that show better frame rates, just more details. http://www.hardocp.com/article/2009/10/19/batman_arkham_asylum_physx_gameplay_review/9
If you look at the last graph you can see that enabling PhysX without a dedicated card your frame rates will actually go down. This review doesn't have the GTX275 with a dedicated card however. If you look at the first chart however you can see the GTX285 with a dedicated card doing around 35-40. (high PhysX setting.) This is 10-15FPS faster then the slower card. I guess your right that a dedicated card will help your frame rates go up. I'd still like to see a review that uses the same GPU. If the GTX275 can get that close, how close would the GTX285 have got?
The next page shows the effects that I mentioned.