I have had good luck with mounting my mini-DV cam to the roll bar with one of these:
http://www.bkauto.com/porsche/r9011.php
Also a good idea to get a wide angle lens. A helmet cam might make for some shakey "sea-sick" footage.
The other cool thing to do is to put a separate microphone in the storage compartment behind the roll bar wrapped in foam or a towel. It gets the sound of the engine without all the wind noise especially if the top is down.