What camera do you use?
A surveillance CCD bullet camera similar to this:
http://www.trademe.co.nz/Sports/Cycling/Other/auction-137187877.htmrecording onto a JXD900 portable media player. I've also made up a microphone with a small amplifier to plug into the audio line-in input for sound. I have had to pull the camera apart and resolder a crystal because of of the leads broke off due to vibration. I've now filled it with silicone sealant so it shouldn't break again, but it's something to be aware of if you try using a security camera on a bike like this. The JXD900 only records at 320x240, but I believe there are other media players (and other security recorder/media player things) that can record line-in input at higher resolution. Or you can also plug it in to a video camera with an av input. If you want a helmet camera though, you're probably better off getting a camera with a wider angle of view because it seems too much like tunnel vision using the standard security camera lense.
The Oregon Scientific ATC2K cameras (
http://www.atc2k.com/) look like a simpler, good cheap option. They've only got a cmos chip and I've heard they can be a bit susceptible to vibration affecting battery/sd card contacts, and in some videos I've seen weird wavy affects that are apparently due to vibrations, but I think you can work around those problems.