Didn't read the whole thread to see if this was already posted or not --
http://www.jacn.org/content/19/5/591.full
Methods: In a counterbalanced, crossover manner, 18 healthy adult males ages 24 to 39, on four separate occasions, consumed water or water plus varying combinations of beverages. Clinical guidelines were used to determine the fluid allowance for each subject. The beverages were carbonated, caffeinated caloric and non-caloric colas and coffee.
Results: Slight body weight loss was observed on all treatments, with an average of 0.30% for all treatments. No differences (p>0.05) among treatments were found for body weight changes or any of the biochemical assays. Biochemical assays conducted on first voids and 24-hour urines included electrolytes, creatine, osmolality and specific gravity. Blood samples were analyzed for hemoglobin, hematocrit, electrolytes, osmolality, urea nitrogen, creatinine and protein.
Conclusions: This preliminary study found no significant differences in the effect of various combinations of beverages on hydration status of healthy adult males. Advising people to disregard caffeinated beverages as part of the daily fluid intake is not substantiated by the results of this study.