Snopes.com does a great job breaking open this health advice snippet that everybody and their grandparents (and especially your grandparents) offer up when nearby people...
Busted: "Caffeinated beverages don't count"
The Center for Human Nutrition released their study results after monitoring the hydration of 18 healthy men drinking water, coffee, caffeinated and caffeine-free sodas, and juices. Their results "found no significant differences" in hydration with any combination
by Daba