Caffeine is a very mild diuretic. In the levels found in coffee and soft drinks it increases fluid output by about 3% at most when compared to water alone. Regular usage drops that 3% to near zero. The chances of you becoming dehydrated by drinking coffee is virtually impossible. Where it can come into play is with energy drinks and energy shots. Energy drinks can contain considerably more caffeine and caffeine like substances than coffee without the water to offset it. Energy shots can be even worse as the have very little water in them. However, becoming dehydrated solely due to energy drinks and shots is still highly improbable. You would more than likely suffer the symptoms of caffeine overdose long before you became dehydrated.
There is a laundry list of studies on caffeine's effect as a diuretic in this link.
http://wiki.answers.com/Q/Is_caffeine_a_diuretic
