Carnival is the only line that has it, and only in cans at $2 a pop.
Still worth carrying on, IMHO. Even in the priciest port, 12 cans is not $24.
(But I still bought cans from CCL when I was too lazy to walk out to get them)
I found this is available on some Royal Caribbean ships: (interesting)
New soda machines on Royal Caribbean cruise ships are a hit - USATODAY.com