That works great if I'm looking for an official video from the Wall Street Journal, but not if I'm watching Fred's Blog Channel. Also, what if I want to edit something? What if I want to tweak the color profile? Add an effect? Oh, so now adobe software can re-sign something, but not the open-source option?

And as pointed out by another comment, you'd get a modified camera and feed it into the sensor, or modify the firmware. And some guy in China will do a clean-room RE and make little video signers.

The bottom line is it's a cool idea, but I don't think it will work, and I don't think it will stop fake anything.