I found this calculator, although there was another one in a post quite awhile ago that I didn't save:
http://www.csgnetwork.com/humanh2owater.html
I have read things that say the "8 glasses a day" thing was set there as it is easy to remember, but it depends on your body weight and other factors. As scorbett wrote, it can be more "fluids" than just water. I have also read that the water in foods plays into it as well.
From strictly a maintenance perspective, the 8 glasses is a great goal because you still need fluid to help feel full as well as proper hydration.
For some reason, I am really struggling getting water in (which I was extremely good about when I was on Phase 1). I might get 20 to 40 oz in of water and maybe another 16 to 32 oz in protein shakes. I do better on days when I have decaf.
It's one of the things I am working on improving. The IP info I received said to continue with the 8 glasses (64 oz) a day in all the phases..