Horses for courses - for high volume sites this would be awesome. My immediate thought was "Great - at last Wikipedia can get rid of the annual begging bowl". It could also be a great solution to the news site paywall debate. I'd love to seamlessly reward the sites I visit often, and if Google can automate this, more power to their elbow.
Also, interesting that it is Google is doing this. It risks reducing their advertising revenue if it takes off too well - I wonder what their cut of revenue is like compared to their advertising business? This model pays them (and the content producer) per impression rather than per click.