N'Gai Croal is one of the few intelligent commentators of the game industry. I agree with him that Nitendo will try to innovate, otherwise there will be no incentive to someone to buy a Wii HD instead of a PS3 or 360, which already have an extensive library of HD titles.
My guess is that Nintendo's hardware team started working at full steam in the Wii successor as soon as they finished the 3DS. Nintendo will avoid competing against itself by releasing two consoles in the same year. As such, as we are getting the 3DS in 2011, they should release their next console by Christmas 2012.
Also, I believe Nintendo is monitoring the Wii sales. They have a strong line up of exclusives for this holiday season: Goldeneye, Donkey Kong Country Returns, and Epic Mickey. If this is not enough to bump Wii sales, then I believe a small upgrade will be release next year as filler before the new console. They might redesign the GPU to be able to upscale the graphics to 1080p and include a HDMI port. Although it would play the same games as the current Wii, it would have HD graphics of some sort.
