With Nintendo now producing and selling paid DLC for some of their games (Fire Emblem, New Super Mario Bros. 2), there have been concerns regarding this for Animal Crossing. Imagine paying real money for more furniture.
That’s no longer a concern though as Iwata has confirmed that after an intensive discussion with the development team a decision has been made to avoid selling DLC/add-on content for a cost. Most likely the free DLC support that Wild World and City Folk had will continue with New Leaf in the future.
The exact quote from Iwata:
For example, some might say that it would be unbelievably profitable to provide paid add-on content for “Animal Crossing: New Leaf,” but we were concerned that a game in which you enjoy yourself more by the power of money would not be suitable, and we decided to avoid such a feature after an intensive discussion with the development team.