Iwata once was quoted to say he "interested in Kidnle-like serivce for DS". It has nothing to do with phones and data plans, as it supposed to be completely free for end-customer (Nintendo would probably pay MNO with share from distribution of digital content) and limited to certain services provided by Nintendo (no Internet, phone functionality etc).
What are benefits of "Kindle-like" 3G connectivity in 3DS? First, 3DS units would be "3G ready", no additional customization needed , it could help Nintendo to battle ignorance of a lot of their users, who aren't that tech-savvy, about downloadable distribution of content more successfully than WiFi connectivity. Furthermore, it's always on and works almost everywhere, while WiFi is limited to certain spots. What are drawbacks? Well, I'm not aware of details, but possibly, price of incorporation of 3G in 3DS, and battery life.
