The Dancer Illusion

Quite a few people have sent the following illusion to me. Basically, as you look at the dancer, she will suddenly shift from moving in a clockwise to anti-clockwise direction. A few sites are suggesting that this is some kind of test of left and right brain thinking, which is obviously rubbish, but it is a nice effect. As yet, I have not been able to find out who actually created it, so if anyone knows, please drop me a line.