Yes, it is off the balance due. You still have to pay the deposit. The on board flyer isn't worded very well regarding this.
They are a great deal for future cruises. They are available for purchase on the Conquest, Freedom, Liberty, Splendor, and Valor. I'm not sure if they added a Future Vacations desk to the Glory during the drydock or not.
If anyone decides to book a cruise on board instead of just buying the certificates, make sure they assign your Travel Agent with the booking if you use one. I did this when they used to have the on board sales on the Destiny. My TA was very helpful with keeping up with the booking and making any adjustments.
Good for you, Tim. I don't mind the trek to the porthole cabins if the price is right. They can get a little "bouncy" though on occasion, at least on the Fantasy class ships. I haven't been in one on the Conquest ships yet.
Edit to remove a defunct file link.