@burcidi-- That's exactly right. Coffee doesn't make the body produce more urine, it just makes urine leave the body faster. It does this by stimulating urination. So that is the reason why coffee doesn't really carry the risk of dehydration like the over-the-counter or prescription diuretics.
@fify-- That's a good question. One reason could be that the type of caffeine in coffee is different than the caffeine in tea. Plus there is just more caffeine in coffee. I can never have coffee at night for this reason, but can have black tea without having any trouble sleeping.
I'm not sure how accurate this is, but I've heard that this diuretic effect of coffee might also be caused, or at least increased by the oils in coffee. Different people might also react to it differently. For example, I don't really drink more water like you do when I have coffee. But coffee does stimulate bowel movements with me, which it may not do for other people.