The best thing I found (for research article I did on another topic) was a literature review that found the diuretic effects of caffeine are short term (I believe on the order of a couple of days) for people who do not otherwise consume caffeine regularly. After that the body naturally adjusts (as you would expect).

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.

The study is:

R.J Maughan."Caffeine ingestion and fluid balance: a review". J Human Nutr. 16:6. p411-420 (2003)

On a separate note, this is why coffee, tea and soda are perfectly viable ways to hydrate yourself for the "8x8" fluid intake recommendation:

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.