Earl Miles (aka. merlinofchaos on drupal.org), posted this demonstration of some JavaScript magic intended for the Drupal panels module.
And check out this by the Man in Blue, Cameron Adams, which is much simpler (and therefore more generally useful) and neatly solves the problem of what to do with a three-column layout in a small browser window. I'm borrowing this technique for a site I'm working on now where the graphic designer has put a lot of fixed-width elements. A List Apart has coined a new buzzword for this sort of thing: Adaptive Layout.
Pretty sad to find myself a web developer fanboy. Excuse me while I put this poster of Jeffrey Zeldman up on my bedroom wall.