The Wii never offered DLC, and everyone hated that. Well, people hate a lot of things about the Wii, but the lack of DLC is certainly one of the items at the top of the list, and it's obvious that the company's direction pushed third-party developers away. Now, though, the Big N hopes to remedy that by allowing for DLC on its consoles moving forward.
According to Nintendo's main man Satoru Iwata, the 3DS and Wii U will most definitely offer DLC functionality. During Nintendo's recent earnings report, the Nintendo CEO made it a point to announce that the 3DS would be allowing for DLC from developers by the end of the year. This is certainly good news, and something that companies who make games for the 3DS will want to keep in mind, as the DLC market is a major part of the video game industry.
Regarding the Wii U, it's obvious Nintendo will have to allow DLC features. After all, if the console is to compete with the Xbox 360 and PlayStation 3, DLC is a must. For Nintendo not to allow developers to create downloadable add-ons for their HD titles would be incredibly foolish, so I think we can all expect similar content to that of Microsoft and Sony's machines.
It should be noted that while Nintendo will embrace the DLC model, the company probably won't be releasing extra purchasable content for its own titles. In other words, don't expect extra downloadable stages for franchises such as Mario or new dungeons for Zelda. I guess Nintendo likes providing an "everything right out of the box" experience with its games. I think most of us can handle that, though. Just give us DLC for third-party titles, and we're set.