Thanks to Darcy O’Neil of Art of Drink,  for providing the information contained in the following article

Caffeine in coffee does not increase dehydration during hangovers.

The effect of caffeinated, non-caffeinated, caloric and non-caloric beverages on hydration.
Grandjean AC, Reimers KJ, Bannick KE, Haven MC. The Center for Human Nutrition, Omaha
(Journal of the American College of Nutrition, October 2000)

Conclusion: 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.

Urinary caffeine after coffee consumption and heat dehydration.
Chambaz A, Meirim I, Décombaz J. Nestlé Research Centre, Nestec Ltd, Lausanne, Switzerland.
(International journal of sport Medicine, July 2001)

Conclusion: These results suggest that the rise in circulating caffeine due to delayed metabolic clearance was partly opposed by a sizeable elimination in sweat. Therefore, heat dehydration did not lead to higher concentration of caffeine in urine after coffee ingestion.

Caffeine, body fluid-electrolyte balance, and exercise performance.
Armstrong LE. Departments of Kinesiology, Nutritional Sciences, and Physiology & Neurobiology, University of Connecticut (International journal of sport nutrition and exercise metabolism, June 2002)

Conclusion: The scientific literature suggests that athletes and recreational enthusiasts will not incur detrimental fluid-electrolyte imbalances if they consume coffee beverages in moderation and eat a typical U.S. diet. Sedentary members of the general public should be at less risk than athletes because their fluid losses via sweating are smaller.

Caffeine ingestion and fluid balance: a review.
Maughan RJ, Griffin J. School of Sport and Exercise Sciences, Loughborough University, Leicestershire, UK
(Journal of human nutrition and dietetics, December 2003)

Conclusion: The most ecologically valid of the published studies offers no support for the suggestion that consumption of caffeine-containing beverages as part of a normal lifestyle leads to fluid loss in excess of the volume ingested or is associated with poor hydration status. Therefore, there would appear to be no clear basis for refraining from caffeine containing drinks in situations where fluid balance might be compromised.

Rehydration with a caffeinated beverage during the nonexercise periods of 3 consecutive days of 2-a-day practices.
Fiala KA, Casa DJ, Roti MW. Department of Kinesiology in the Neag School of Education at the University of Connecticut (International journal of sport nutrition and exercise metabolism, August 2004)

Conclusion: In summary, there is little evidence to suggest that the use of beverages containing caffeine during nonexercise might hinder hydration status.