whoisgregg - 10:07 pm on Apr 1, 2009 (gmt 0)

screen shots as a small GIF. Would it not be possible to automate the comparison, detecting if the content of the page has changed significantly since the last crawl?

This would work except for rotating or changing content. From a pure "how many pixels have changed" viewpoint, a blog home page changes dramatically when a few new entries have been posted. A large ad block would have completely new pixels on each page load. And, even just the entire page shifting down by one pixel would cause a pixel for pixel comparison to be completely different.