Caffeine is a dehydrant.

Here's my own experience:
after having drank a lot of regular coffee during several days, I
noticed that my skin was getting dryer and dryer.
I tried to switch to deca: appart from the fact that I wasn't getting
any stimulating side effect, my skin remained the same.
Then I started to sswitch to plain water instead of coffee for a few
days: my skin got better in 2 days.
So I think that the diuretic part of coffee may not be caffeine.

H u eyduck

I feel dehydrated if I drink coffee (or beer) without a glass of water too.  
I have a very weird thing about me, if I get dehydrated, or just regular
thirsty, I feel gaggy at the back of my throat, and it makes me burp.  I
think I'm burping to sort of dislodge the gaggy feeling. If I drink coffee
without a glass of water, I burp quite a lot.  I know, gross, but what the
hey.

Depends. If you never drink coffee and then one day you drink a couple of
pots, then yes you might end up slightly dehydrated, although not pillar
of salt time.

If your body is acclimated to caffeine it's not dehydrating.

But, don't take my word on it, here's a link to the study:

http://www.jacn.org/cgi/reprint/19/5/591.pdf

Empirically, I used to drink diet coke almost exclusively. I was not
appreciably dehydrated.

Martha

The following is an excerpt from an article about water in the Los
Angeles Times on November 20, 2000.

The way it's almost always stated, in books, magazines and newspapers,
the 8-by-8 rule specifically discounts caffeinated beverages, such as
coffee. This is flat wrong. Caffeine does cause a loss of water, but
only a fraction of what you're adding by drinking the beverage. In
people who don't regularly consume caffeine, for example, researchers
say that a cup of java actually adds about two-thirds the amount of
hydrating fluid that's in a cup of water.
That is to say, one cup of coffee equals about two-thirds a cup of
water--if you're not a regular caffeine drinker.
Regular coffee and tea drinkers become accustomed to caffeine and lose
little, if any, fluid. In a study published in the October issue of
the Journal of the American College of Nutrition, researchers at the
Center for Human Nutrition in Omaha measured how different
combinations of water, coffee and caffeinated sodas affected the
hydration status of 18 healthy adults who drink caffeinated beverages
routinely.
"We found no significant differences at all," says nutritionist Ann
Grandjean, the study's lead author. "The purpose of the study was to
find out if caffeine is dehydrating in healthy people who are drinking
normal amounts of it. It is not."
The same goes for tea, juice, milk and caffeinated sodas: One glass
provides about the same amount of hydrating fluid as a glass of water.
The only common drinks that produce a net loss of fluids are those
containing alcohol--and usually it takes more than one of those to
cause noticeable dehydration, doctors say.