Candice brought up a good point that I didn't mention before regarding the self serve soda fountains. These are for the passengers that have purchased the soda package. As stated, there is a chip in the mug which will activate the machine, so other passengers don't try to get the fountain soda that have not paid for the package. However, there were a couple issues that I found with that.
The first one was with the wind. The self serve fountain is on the pool deck, if I'm remembering right. At least, that's where it was on the Freedom of the Seas. Because of the wind at sea, the soda wasn't going straight down into the mug, but rather to the side. The other thing is that the mug has to be placed just so in order for the fountain to work. I thought I had positioned it correctly at first, but found I had to adjust it slightly. I found it easier just to go to a bar and have them fill it. They may ask to see your SeaPass card, to make sure you purchased the soda package.
2007 - Majesty of the Seas
2009 - Enchantment of the Seas
2010 - Freedom of the Seas