Just a quick caveat to anyone planning on downloading the just released Kingpin version 1.1 patch (story): we've gotten several reports that you will not be able to use any previously saved games, as well as many reports of people randomly being kicked from multiplayer servers. Also, we received details on how to use the vehicles in Kingpin from Daniel "Milamber" Woods of WA Gaming Zone:

I was just fiddling around with Kingpin and found out how you can test the very cool vehicle code that didn't quite make it into Kingpin. As you probably know, before being hired by Xatrix, Ryan "Ridah" Feltrin coded the immensely popular Q1 TC, Quake rally. So, the vehicle code is pretty damn special. Anyways, start the game using +set developer 1 and the bring down the console and type g_vehicle_test 1 and for a really authentic feel, turn on the rear-view mirror by typing cl_rearviewmirror 1. Now map one of the DM levels, kpdm1 works fine, and try being, from what I can gather, a motorbike, complete with fairly realistic physics and sounds.