One thing I did find for the mac which is worth checking out is SecuritySpy. It has a motion detection feature. I have tested it on my Intel Quad Mac Pro and it seems to work well with my iSight. I think I will use this software when I setup my mac car, however I need to do more investigating on overlaying the speed, etc.
http://www.securityspy.com/
I do wonder why you guys are using GPS speed rather than tapping into your vehicle's central computer to get the actual vehicle speed. You would think that would stand up better in court than a GPS speed.
Anyway I thought I would share this information
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Oh this also supports a lot of cameras so you can use it for your home or in your car. What about the super paranoid who want a camera on all sides of their car for accident reasons

Also has the ability to upload the video's to an FTP server. Write a little script that makes airport connect to any wifi network it detects and upload. I just hope it would resume the upload because if the car is stolen they would probably hit several wifi networks before coming to a stop.
Nice Anti-Theft Device catching them in the act

oh and you would have turn by turn directions lol.
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