They Didn't announce cross platform online. 3ds will NOT have online. they Announced Cross-REGION online. meaning that people in North america can play with People in Europe. you can play locally with 1 Wii U and up to 3 3ds' but only locally, not online.
Rejera, you are only half correct - they are supposed to release in a few days' time something they call the Packet Relay Tool, which will allow you to take the 3DS version online. 2 Caveats - you will need a Wii/WiiU LAN Adapter, and I think you won't be able to play the WiiU at the same time you use it for this purpose.