You are actually fairly limited in your ability to create a revision to your hardware mid generation ...
Size and stlye changes are an obvious possibility and we could see a mini-Wii if Nintendo thought people would be interested; as they move to the 65nm (and eventual 45nm) process the CPU and GPU will produce less heat and can therefore be put into a smaller area without consern for overheating, and Nintendo could use a laptop optical drive.
Trivial resolution increases through upscaling are always a possibility but are generally not what people refer to as a Wii HD ...
Hypothetically speaking, if Nintendo abstracted a certain portion of lower level graphical functions (mainly dealing with output resolution and textures) they could produce a Wii system that had a higher output resolution, upscaled textures while maintaining bidirectional compatibility; essentially they would be able to enhance games in hardware the way software emulators have for years. This would be the least likely of the possible implementations.
Nintendo simply will not release a system that performs like the PS3/XBox 360 in 2009 because it will hurt them. They will be seen as abandoning the Wii after only 3 years which will annoy anyone who bought the Wii as well as developers who just got on board with the Wii. Consider both the person who bought the Wii for $200 in late 20008 and the developer who finally finished their PS3/XBox 360 title in Q4 2008 and began a Wii project; the gamer will not be happy because he will have to buy new hardware to play upcomming games, and the developer will not be happy because they will be expected to either increase development costs to release on a platform with a tiny userbase or to release a game to a dead system.
The fact of the matter is that people made similar claims about Nintendo releasing a follow up to the Gameboy Advance or Nintendo DS in 2006/2007 and we haven't heard anything; Nintendo will probably announce a new platform to replace the Nintendo DS at TGS 2008 which will be released in 2009.