If you, like many other ad guys, dig the optical illusion in this ad (and in at least eight others), or similar illusions of spinning wheels (seven different ads) or cog wheels, you will love the ones brought to you by MIT's Computation Visual Cognition Lab . They are much less known than the ones above by Akiyoshi Kitaoka, are arguably more visually stunning and lend themselves to advertising.
Click on the image to zoom in, and move back to see the faces change places. If you want to sound impressive when showing this off to your friends, here's the schpiel: "This illusion illustrates the ability of the visual system to separate information coming from different spatial frequency channels." Here. Now you know.
UPDATE (27 OCT 05): BoingBoing runs a how-to about making your own illusions in Photoshop and apply them to t-shirts, for example.