We've integrated bullet into our web-based 3D content system called Hypercosm. Like many others, if the forums are any indication, we struggle quite a bit getting the tuning right on vehicles. I created a little utility to make that easier, and I thought I'd share. http://www.hypercosm.com/private/CarTuner/CarTuner.zip
It simply loads a vehicle into a little test course and provides sliders to adjust the attributes of the suspension etc. as the applet runs, so you can tweak to your heart's content until you get the performance out of the car that you want. It hasn't been real thoroughly tested, but it's been useful for us.
Presumably if someone wanted to try to grok our XML format you could put your own vehicle definition in there. See the car_physics.xml (physics definition) and car.xml (geometry definition) files in the 'applet' folder if you want to try.
To view it you'll need to install the player (ActiveX browser plugin - installer included in the ZIP - it's all sandboxy and safe).
If you're curious about Hypercosm, here's our Google Code page:http://code.google.com/p/hypercosm/