Amazing Optical Illusion

A friend forwarded this amazing optical illusion to us earlier today. Now we've seen a number of 'illusions' before (remember staring at the pattern on the newspaper comics page?), but never one quite like this. We're sure there's a scientific explanation for what causes the illusion of motion, but we've no idea what it is. So, if you know, feel free to comment. We'd certainly like to know how this works!