I believe Nintendo will announce the Wii successor after the DS successor. As such, it should be at least two years away. Still, I wouldn't be surprised if they release a new Wii SKU capable of upscaling the graphic output to 1080p. This would be cheap, it would address some concerns about playing the Wii in HD tv sets although it wouldn't please the hardcore gamers, as the polygon count and textures' size would still remain limited.