Uru: ABM was originally developed using Havok physics, but the licensing cost was too great for Uru Live, so they switched engine to the free-to-use PhysX library. That I am aware, there is no "Free" version of Havok that can be used on anything other than Windows, whereas PhysX can be used free on Windows and OSX, provided the project license doesn't attempt to impose it's license on third-party libraries (Like the GPL does).
Also, PhysX can be hardware accelerated with a PPU or an NVidia graphics card, which is something Havok doesn't do.
I have no connection to PhysX here, just saying how things are at the moment. I'm certain that, if you wanted to, you could rip out the PhysX engine and put a Havok one back in, or even throw in ODE (Which is what Myst V used), but at the end of the day, what we're getting to start with is a game with a PhysX engine built in, and unless there is a major technical reason for it, there seems little point in changing it for changing it's sake.
That's my take on it, anyways.