The Institute of Medicine released revised hydration guidelines in April of 2004 which concluded there is no evidence that moderate intake of caffeine results in "body water deficits".
This is not to say caffeine in and of itself is not a diuretic – it is, in the sense that it increases urine production, especially when consumed in high doses.
Keep in mind, though, that the same thing can be said about water. You will urinate at a higher volume on the days you drink ten, rather than five, glasses of water.