I'm thinking along the same lines as you, Tut. But I'm not into tearing my doors apart to run the wiring. You can mount the cameras in the cabin easier. I was going to place mine in those small triangle shaped rear windows that don't roll down - or the upper corners of the rear windshield. Mount them at the top of the window, so they're out of sight of thieves and such.
I would also like to mount one in the far right upper corner of my front windshield, so I can see "around" cars that are loitering in the passing lane. (man, I hate that!)
My problem is that I haven't found any free software that will handle more than one camera input, and I'm up to 3 cameras! Also I bought a couple cheap ($10) web cams to play with and WinXP will only recognize one camera at a time. It confuses the same make/model camera's and thinks there is only one plugged in.