If you own a copy of Monster Hunter 3 Ultimate on both the Wii U and Nintendo 3DS, you will be able to share data between games. The trailer specifically shows how you trade items between games. You will also be able to play with other Wii U owners online, or play with Nintendo 3DS owners. Definitely an exciting feature for the biggest Monster Hunter fans.
Monster Hunter 3 Ultimate will be available on Wii U and Nintendo 3DS sometime during March 2013.