I think that Nintendo may reveal a Wii successor at next year's E3 for one main reason: The 3DS is going to make the Wii look dated. I mean more than it does already. The portable system should never out-perform the home console, and the 3DS is going to do just that--especially with the rumored tech inside out-performing the Wii.
I think the next Wii is coming sooner than the next Xbox or Playstation. Nintendo is forcing themselves to upgrade.