From Google's point of view, especially since CPM ads have been introduced, this might fall under the category of generating false impressions. My advice would be to stay clear!
On traffic exchanges, if you look as you surf, it seems to be that the first time a site is shown to you, google adsense is available. But from then on, when the site comes up again, no google! Again, google has systems in place to check where your site is getting all those hits and clicks. If you do advertise on traffic exchanges, use a splash page and just get people to go to your site off the te. The Iowa Dawg