Excellent news! I work for a very large publisher in Canada that uses WordPress as its CMS of choice. Because we use Akamai and shrug at bandwidth costs, I don't have much of an argument there to use of this service, but shrinking all images on a page by 25% to 50% is going to have a noticeable impact on page rendering time.

The plug-in would have to make the process completely transparent, though - it should happen as part of the normal WordPress upload workflow, with not even one extra step for users to have to take.