This is technically an average, as the actual recommendation is really based on your weight (so bigger folks like myself actually need more). The best number I can find is about 35 mL/kg, which translates to about .5 oz/pound. So someone around 170lbs would “need” about 80 or so ounces, while someone closer to 200 pounds would go for 100 ounces.
What I found cool was that even in the “original” source for the recommendation, it mentions that most of the fluid intake would come from food, but that people ignored that part. From a very comprehensive review of the available literature done by Heinz Valtin of Dartmouth:
According to J. Papai (65), P. Thomas has suggested a different origin for 8 × 8. Thomas reminds us that in 1945 the Food and Nutrition Board of the National Research Council wrote (31): “A suitable allowance of water for adults is 2.5 liters daily in most instances. An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods.”
Thomas suggests that the last sentence was not heeded, and the recommendation was therefore erroneously interpreted as eight glasses of water to be drunk each day.
The current recommendation is in fact to consume about about 2.5-4L of water a day, of which at least 20% will come from food and that soft drinks and mild alcohol count toward (discussed below). If you look for it, there is also some controversy that the suggested amount is higher than necessary.
So we’re supposed to have at least 2L of water a day, but that has to come from plain drinking water. Or does it?
The AIs provided are for total water in temperate climates. All sources can contribute to total water needs: beverages (including tea, coffee, juices, sodas, and drinking water) and moisture found in foods. Moisture in food accounts for about 20% of total water intake. Thirst and consumption of beverages at meals are adequate to maintain hydration.
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. The across-treatment weight loss observed, when combined with data on fluid-disease relationships, suggests that optimal fluid intake may be higher than common recommendations. Further research is needed to confirm these results and to explore optimal fluid intake for healthy individuals.
Additionally, in a controlled study where one group consumed a standard diet (except with no water, but other beverages) and another had a diet including plain water they concluded “Inclusion of plain drinking water compared to exclusion of plain drinking water in the diet did not affect the markers of hydration used in this study.“
The exception appears to be (at least) for those who are just now restarting to drink caffeine after having abstained for a week or so. Basically the body very quickly adapts to counteract the diuretic effects. In a literature review published in the Journal of Nutrition and Dietetics, the authors found :
The available literature suggests that acute ingestion of caffeine in large doses (at least 250–300 mg, equivalent to the amount found in 2–3 cups of coffee or 5–8 cups of tea) results in a short-term stimulation of urine output in individuals who have been deprived of caffeine for a period of days or weeks. A profound tolerance to the diuretic and other effects of caffeine develops, however, and the actions are much diminished in individuals who regularly consume tea or coffee. Doses of caffeine equivalent to the amount normally found in standard servings of tea, coffee and carbonated soft drinks appear to have no diuretic action.
In terms of alcohol, in a study looking at fluid balance recovery after exercise, “there appears to be no difference in recovery from dehydration whether the rehydration beverage is alcohol free or contains up to 2% alcohol, but drinks containing 4% alcohol tend to delay the recovery process.” 
If you’re interested in some of the other dietary guidelines for vitamins in nutrients, a good jumping off point is the USDA’s National Agricultural Library DRI tables.
1 Institute of Medicine’s Food and Nutrition Board. “Dietary Reference Intakes : Electrolytes and Water”. http://www.iom.edu/Global/News%20Announcements/~/media/442A08B899F44DF9AAD083D86164C75B.ashx Visited 11/18/2009
2 Ann C. Grandjean, EdD, FACN, CNS, Kristin J. Reimers, RD, MS, Mary C. Haven, MS and Gary L. Curtis, PhD. “The Effect on Hydration of Two Diets, One with and One without Plain Water”. J of Am Coll Nutr. Vol 22, No. 2, 165-173 (2003). http://www.jacn.org/cgi/content/full/22/2/165
3 Valtin, Heinz. “‘Drink at least eight glasses of water a day.’ Really? Is there scientific evidence for ‘8 × 8’? Am J Physiol Regul Integr Comp Physiol 283: R993-R1004, 2002. First published August 8, 2002; doi:10.1152/ajpregu.00365.2002 – I would highly recommend my readers take a look at this if they want a more comprehensive look into this topic.
5 Ann C. Grandjean, EdD, FACN, CNS et al.”The Effect of Caffeinated, Non-Caffeinated, Caloric and Non-Caloric Beverages on Hydration”. Journal of the American College of Nutrition, Vol. 19, No. 5, 591-600 (2000). http://www.jacn.org/cgi/content/abstract/19/5/591
6 Ann C. Grandjean, EdD, FACN, CNS, Kristin J. Reimers, RD, MS, Mary C. Haven, MS and Gary L. Curtis, PhD. “The Effect on Hydration of Two Diets, One with and One without Plain Water”. J of Am Coll Nutr. Vol 22, No. 2, 165-173 (2003). http://www.jacn.org/cgi/content/full/22/2/165
7 R.J Maughan.”Caffeine ingestion and fluid balance: a review”. J Human Nutr. 16:6. p411-420 (2003). http://www3.interscience.wiley.com/journal/118888724/abstract
8 Susan M. Shirreffs and Ronald J. Maughan.”Restoration of fluid balance after exercise-induced dehydration: effects of alcohol consumption”. Journal of Applied Physiology. Vol. 83, No. 4, pp. 1152-1158, October 1997. http://jap.physiology.org/cgi/content/abstract/83/4/1152