Get a decent webcam, Logitek 4000 springs to mind, then a notebook and download VirtualDub. Put the notebook in a backpack, gaffa the webcam to your helmet, have someone help you start recording. The main problems with this are, the webcam footage will be 15fps and if you use full resolution (640 x 480) it will generate an enormous file. Try one gig for 4 minutes. Also, you may have to fool the notebook so you can close the lid without it going into "hibernation" mode. Advantages are, very portable, you will already have a digital file and importantly, its cheap!