IGN is now reporting on this, and hasn't labeled it as a rumor, for what that is worth. Although there is still speculation and unknowns.
http://wii.ign.com/articles/116/1161875p1.html
One of the sources says that the Wii successor will be "significantly" more powerful than the PS3 and XBox 360. Also, sources say that Nintendo wants to "recapture the hardcore market". Also, there will be a new controller unveiled. Rumor is that it'll have a touch screen. Hmm, we'll see about that last one.
Also, the console is said to support 1080p and backwards compatibility with Wii games. IGN says Nintendo will announce something this month, and will unveil it at E3.
Lots of rumored information. We'll see if Nintendo will indeed make an announcement about this console later this month. I would bet on an unveiling at E3, though. Different sources have said that will happen.