I've used an I/O Port camera mount on the roll bar (also used an old radiator hose to protect the finish on the roll bar) with great success. Works perfectly! Check it out:
http://www.ioportracing.com/Merchant...gory_Code=IOPP
BTW, though the GoPro does provide hi quality video, I see the lack of external mic input jack and external power input as being show-stopper qualities for me, IMHO. These are two qualities that I see as key to creating good in-car video.