Wii 2 is indeed in development, and the confirmation has come from none other than Mario Creator "Shigeru Miyamoto".
In an interview to Edge, Shigeru Miyamoto was quizzed about Wii 2 and was pressed for some concrete detail on it, to this he replied:
"Don't ask!, "Even when the Wii launched we were developing new hardware, work on 3DS had already started. It's a matter of when we announce it."
When asked if Nintendo will make an official announcement at E3 2011, Miyamoto said: "Please wait. Be patient until we decide."