The wording in the Cruise Planner was changed a few days later to made it clearer that the upgrade price was $10/day and not just $10 for the entire cruise. More importantly, when you added it to your cart, it was corrected to show the $10 price being multiplied by the number of nights of your cruise (before it was not multiplied when you checked out).
There were those of us that pounced on it and purchased the upgrade for $10, and of those that have taken their cruise, they reported that it was indeed for the whole cruise ($10). I'm waiting for my December cruise.
When the error was first reported, there was a lengthy debate on this forum on what was actually purchased (e.g. you bought an upgrade for one day) whether it was really a mistake (that it must be $10/day and that you'll be charged for the balance later), or if it was indeed a mistake whether it would be honored (e.g. the iLounge will tell you that it's a mistake and you need to pay up).