Agree with DaRev and LinkVPit.
Likely you'll have an empty Miiverse when you don't have internet or when you first boot up the WiiU, then if there's a connection, it'll download whatever is necessary to properly run it. At least it'll prompt you or something, asking you if the WiiU is allowed to set up the Miiverse's internet abilities.
I think.
I don't think they'll have some kind of alternate off-line homescreen and you have to physically enable the internet, start up the store, find Miiverse in the store, download it and install it.







