Wonder if any car drivers have started doing the same, for a similar reason as you ;-)
Seems sensible. I know some truckers leave cameras running to collect evidence in the event of a collision. In fact, I once heard of a system that recorded video on a short loop until it detected a collision (via an accelerometer) - but I'm not entirely sure what the advantage of that is, given the low cost of storage media.
I have an old camera phone that I keep on meaning to strap under my saddle to face backwards. The resolution isn't up to much, but it will at least show what was going on behind me in the event of an accident (I already have a forward-facing camera mounted on the handlebars).
