Bullet gets funded by Sony Computer Entertainment. They hired Erwin Coumans (a former Havok employee) to work full time on it.

If Gamestudio has PhysX built-in like currently ODE (directly in the acknex.dll), won´t every project need the driver then no matter if it uses physics or not? That´s unacceptable to me.

I think if you absolutely want to use PhysX then the dual physics engine solution like Fastlane69 mentioned would be best. Replace ODE with Bullet in all editions and offer PhysX for the professional edition as an add-on. There already are physics engine abstractions layers out there so maybe this wouldn´t be that much work. It also would be one more good incentive for the professional edition again (at least for people slavishly following marketing hype :p).