Just FYI, it doesn't work in FireFox (1.0.2) - but we've used that type of coding to anchor Google AdSense blocks on a few client sites. If done smoothly, it doesn't seem to annoy visitors. For example, we have an AdSense skyscraper on the far right that tracks with the user as they scroll down....
It's less annoying when it's anchored to the top of the page, as opposed to the bottom (more jumpy, especially with smaller content blocks inside...
Rob