Regarding the idea of modifying video, CBS inserted fake billboards in their countdown to the Year 2000 during that year's New Years Celebration. It didn't go over well, so it will be interesting to see what people think.

http://query.nytimes.com/gst/fullpage.html?res=9C06E7DB173AF...

I've seen this ability to replace a moving rectangle in a video with other content in 3D software before. With high definition video, I imagine this process is more accurate than ever.

I think a bigger question is how to get someone to pay you to add fake ads to your YouTube submissions, and how to track that somebody is actually doing this, and for how many seconds. That could get very interesting.

Also, related:

Related, the other side: how to detect that a video WAS modified:

http://news.cnet.com/Are-fake-videos-next/2100-1008_3-611344...