iIR: img Image Replacement appears to be a great way to provide images for both the on-screen and print versions of a site, without adding additional markup, nor sacrificing usability and accessibility when images are disabled.
The one thing I’m not so keen on is the fact that you are forcing the user to download the print version of the image before they have decided they want to print the page - or, more importantly if they want to print the page.
A discussion of the concept is running at the associated blog entry.