Bullet uses the same license as Irrlicht, the Zlib license. That is a much better license than the closed source PhysX license.
The vehicle example in the Bullet SDK is meant as a starting point, but you are expected to write most of the vehicle implementation yourself: most commercial games use their own vehicle implementation, even if they use Havok, PhysX, Bullet or ODE for the rigid body dynamics.
In case you want to start with the Bullet btRaycastVehicle, I recommend reading this doc and using the forums for further vehicle help:
http://docs.google.com/Doc?docid=0AXVUZ ... 4Zmo&hl=en
(doc was written by developer of Hotwheels
and Madagascar Kartz
. There is also an open source game, http://www.extremetuxracer.com
, using Bullet and Irrlicht.