Pretty interesting. I wouldn't have guessed it slightly reduced sodium reuptake. Some folks really love (lightly) salted coffee, I'm sure that would solve that issue of one was concerned about it. Doesn't seem to be something to worry about though.Here's an interesting paper on the diuretic topic (this is just the abstract. Click to get the paper, which is in French. Google translate does a good job):
Caffeine is an alkaloid which belongs to the family of methylxanthines and is present in beverages, food and drugs. Caffeine competitively antagonizes the adenosine receptors (AR), which are G protein-coupled receptors largely distributed throughout the body, including brain, heart, vessels and...pubmed.ncbi.nlm.nih.gov
I think the key takeaway beyond what has already been discussed is if you're not used to caffeine consumption, coffee is more of a diuretic than for a regular consumer.