If you want to mount it on the bike rather than the helmet, the Cerevellum Hindsight gives you a rearview camera display -
http://www.cerevellum.com/hindsight.aspx

With -
Video Recording
The systems records video in a continuous 5-minute loop, stopping automatically if the bike is laid over.

I don't know if this is great or not though - I might want it to keep recording after the accident to record what was said.