I bet the cameras' companies will start automatically uploading the real footage to their servers for attestation, and allow the camera owners to get those links, so people will just add that link on YouTube or whatever and say "See, its real, Sony vouches for it", heck maybe they will make their buyers to sign up with YouTube and do it for them.
How on top of security do you think all the camera manufacturers are going to be? That is, how long until people can sign videos that were not, in fact, shot with their camera?
Perhaps that could work in certain situations, but you don't even necessarily need digital signatures for that. A link to a reputable news site claiming they've verified the footage as real would be good enough in like 95% of cases, people just don't bother to check.
You'd also need close to 100% adoption for this to be effective, otherwise people will just assume the fakes were recorded with one of the cameras that doesn't have that feature, or that they didn't bother to upload the raw footage anywhere.