Do not do this to random people's websites. Google is one of an enlightened few. You will be surprised how easy it is to piss people off just by looking for cross-site scripting; something innocuous you do is going to cause popups to appear for all their customers, and they're going to go ballistic.
Just for the record that Google announced that they allow certain checks - http://googleonlinesecurity.blogspot.com/2010/11/rewarding-w... (actually encourage and reward - better than people publicly releasing these for sure) but even for them many vulnerabilities are still out of scope. Otherwise as you stated this is illegal in almost all countries.
At the risk of being downvoted, here's a "conspiracy theory." Why did the ad block for firefox recently stop working for youtube and google sponsored video ads? Personally, firefox's superior ad block was the last thing preventing me from defaulting chrome. Maybe google engineers figured out a way around the ad block, who knows. Regardless, does anyone know an ad block that still takes care of the video ads?
I had this happen to me several weeks ago, it turned out the blocking list I had subscribed to (EasyList) got switched with the new default "Fanboy's List". I had switched back and haven't been plagued by video ads since.
People have gotten into legal trouble doing this.