To be fair, an "impression" isn't really a great metric on a page that people sit on for >1 minute. The guidelines we worked under at my previous job basically

To be fair, an "impression" isn't really a great metric on a page that people sit on for >1 minute. The guidelines we worked under at my previous job basically said "if a user action changes a substantial portion of the page, it qualifies as an impression". It seemed pretty fair to me at the time. Advertisers (especially with rich media) get ridiculous interaction metrics from their creative, they know what people are doing with their ads. Lots of them are happy if an ad sticks on a page for half a minute and is then replaced.