A Wii HD will not be a new console, just like DSi is not a new console...that said I think we'll see the same order of this gen: MS ---> Nintendo ---> Sony.
IMO Microsoft will launch the new console with Natal included or bundled, and that will be one of the main attractives of the new systems;
Nintendo will include HD, possibly include also some periphals who had success this gen (Balance Board, Wii Motion Plus, hopefully Wii Speak, even if it's not successful...) and something unthinkable now, like the Wii in the past gen.
Sony I don't know, releasing a new system would not be an intelligent thing, they have yet to make profit, the console can still be cut in price, they're the only to have the Blue Ray Disk incorporated, and the Online system can still grow...maybe they'll release some new SKUs, but there's the possibility for Sony to continue for some years in the next generation with the PS3 (remember, 10 years plan...).