I'm not citing Penn, per se. He interviewed several doctors and nutritionists, they were the ones who stated that additional water in the diet is largely unneeded, and that rumors of chronic dehydration in the general populace are wrong.

Also, Snopes has addressed the topic: http://snopes.com/medical/myths/8glasses.asp

Believe what you'd like, but don't try to mislead other people with it.

(Have I ever eaten a solid meal without drinking fluids? fuck yes. are you kidding?)