I did some quick experiment this afternoon, integrating Bullet into one of the default Irrlicht graphics examples (Quake 3 Map), and I'm quite impressed with the ease of use and quality of Irrlicht. Both Irrlicht and Bullet use the liberal ZLib license. Note that the Bullet character controller is under development and not as mature as the Irrlicht controller yet, but it has some interaction with the rigid bodies.
Added the Bullet BspConverter to load the .bsp map
Hook up an event receiver to intercept mouse / keyboard for Bullet character controller
Glad to see that you enjoy using irrlicht!
I myself have used bullet a whole bunch in irrlicht and bullets simplicity when combined with irrlicht makes it really simple.
Thanks for the example, this should help a good number of people!