Look closely. Or not too closely ... and you'll see an optical illusion on the album sleeves of the band Soulwax. (Need a hint? Don't look directly at your monitor - view the picture above at an angle).
The Monkey Buddha has one more example: Link | Another Phantom Message - Thanks Paul Micarelli!
Update 8/1/08 - Sleevage featured this first (they also have many more cool album covers) - thanks Ashley Ringrose!
But I found it easiest to slide the glasses to the tip of my nose and look through them.