I guess if its just an 'evidence' based device, then anything would do with a half decent res (wouldn't have to be full 1080p), and capacity to film your entire commute. Get a couple of cheap Muvi cams and you could do front and rear!!
Kris Zentek has got a Garmin Virb. I think there may be a couple with GoPros and the like.
Think Nick has got a cam as well, but don't know what it is....
I can't recommend the virb enough for a bike can. Marginally lower quality than a gopro black edition ( and I mean marginal!) But far superior in terms of on bike features. Especially if you use an egde 510 or 810.
You might also want to take a look at the drift HD - it is a similar form factor to the virb, but gas a unique 5 mjn record buffer. Basically you can set it to buffer record continually, and at the press of a button it immediately saves the last 5 minutes. Very good for capturing awesome riding, or dangerous drivers.
I'm going to the forest tomorrow, I'll bring my GoPro you can borrow if you want.
You can get some really tiny ones quite cheap, prob good enough if all you want is evidence of idiocy and a numberplate. Like this from Amazon.
http://www.amazon.co.uk/Veho-VCC-003-Muvi-Micro-Camcorder/dp/B0029631V I"> http://www.amazon.co.uk/Veho-VCC-003-Muvi-Micro-Camcorder/dp/B0029631VI
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