I remember seeing an article in the newspaper about a woman who hasn't washed her hair for about 4-5 years. Well, she doesn't wash it with shampoo anyway. Something about how the grease in your hair can act as a shampoo when washed with water.
I tried it once for the summer holidays. Didn't work out too well... It didn't look like I hadn't washed my hair in that amount of time, but it didn't work how I wanted it to.