Nintendo have today given more details regarding online play in Monster Hunter 3 Ultimate for 3DS. It's quite a complicated process, but Nintendo have made it clear.
To enable online multiplayer in the 3DS version of the game, you must first purchase a special LAN adapter for your Wii U, and then download a special application that will be available on the Wii U eShop. This will make the 3DS version of the game playable online.
These details are of course for the Japanese version of the game, so we don't know how the process with differ in Europe and North America, or if the 3DS version of the game will be playable online at all. What do you think?