I very nearly did the same thing, then I realised I already had a high-res camera with GPS, inbuilt maps and 3G data connection so I got this free app for my phone : https://play.google.com/store/apps/details?id=com.hovans.autoguard&hl=en_GB along with a 32gb SD card
Only problem the phone has to use a screen mount and they keep breaking under the strain.
doesnt film the cars comming up your rear though does it,and then trying to blame you for braking needlessly.
and how does it work when your using the sat nav on the phone?
I'm more worried about idiots in front of me and crash-4-cash scammers. It records speed via GPS and detects impact, so if someone does hit me in the rear the app would prove my speed and the video capture of the road in front would prove if I suddenly hit the brakes or not.
I think if you get the paid version it will carry on recording in the background when you're using the satnav. Personally I don't often use my phone for satnav so it hasn't been a problem.