Virals are great. They break rules that traditional media imposes on advertisers. You can do sex, violence, racism to an extent and anything that would be a no go in national TV. Actually the more shocking the faster it spreads. The more your audience is outraged the better it works. Whether it does any good to your brand is a different question. You can see some examples right on this site.
They usually spread through personal channels. Mainly email. Friends, collegues and relatives are sending them to each other. Therefore such messages have a personal attachment to them by default. If it's good it becomes a topic of discussion.
However no matter how effective tool viral marketing is, without a good measure of it's success there is little chance a major client would buy into the idea of doing a viral ad. This is where Viralchart comes into the picture. You can add a sprite to your viral videos or games that will enable Viralchart to track your video being played, no matter if it's posted on a blog like Ads of the World or opened as an email attachment.
Unfortunately it won't work with press ads and I don't see how it can be done effectively. One simple way would be to embed the images into a flash or Quicktime movie. Of course it will only last until someone makes a screen grab.