Basically it comes down to this

No matter what people say about ATI owning only 25% of the discrete GPU market, it's still 25% of your possible base. Developers will never alienate 25% of their potential customer base unless they want to be removed by their bosses and shareholders.

This was why previous games w/ PhysX effects that required a PhysX card were all optional effects and were more for show than practical use - because no developer is stupid enough to shoot themselves in the foot by requiring people to upgrade/change video cards when the game runs fine otherwise.

It's why PhysX was labeled a failure long before Nvidia bought it - because it was a catch-22 situation. You force people to use the PhysX card and then no one buys the game unless they have the money to buy that + the hardware. You remove the need for the PhysX card but then you can't enable the sophisticated cool stuff that could make a game. PC game developers are very against taking risks in this day and age and they always tend to play it safe (except Crytek it seems, but look at their poor sales figures on Crysis).

And besides, with Intel still pushing multi-cores + their own discrete GPU Larrabee, there is no doubt in my mind that Intel is pushing hard to ensure that they can't be shut out.