A dash cam is an excellent idea (and, one of these days, I'm going to get off my lazy, ample asset and install one). A dash cam is an excellent way to prove if someone ahead of you is the cause of an accident.
Where I live, unless one has positive proof otherwise, the person hitting someone behind is automatically considered to be at fault. However, if someone suddenly changes lanes in front of you and jams on the brakes, causing you to hit him, a dash cam video will allow you to prove you weren't at fault. If some moron passing you cuts back into your lane a bit short and clips the corner of your vehicle, then books, you will have a video record of the accident, including the moron's license plate.
Recently, I saw an idiot almost wipe out a motorcycle by changing lanes in a turn. I wished I had had a dash cam running so I could have turned the video over to the gendarmes. There have been plenty of other times I wished I had one, such as when stuck behind an inconsiderate slowpoke that refused to use the pull outs to let the lineup of cars following to pass.
If a cell phone can be safely secured to a dash and require no intervention to operate, I see no reason to believe it wouldn't be a good idea.