I think it goes without saying that there'll be Wii HD.
I think it's a matter of when it'll be released and how much will change between the Wii and Wii HD. Will the next Wii include new different controllers, or continue with the same Wii controller set-up and just upgrade graphics.
Also, what other possible features could it have. Nintendo will also need to compete with Natal coming out later this year, and SONY's 3D and motion control.
The potential success of each of these additional features can be argued about, but each of HD consoles are adding features, and the Wii isn't. It doesn't mean the Wii isn't fun... but the Wii needs to maintain or increase sales (same as the HD consoles) but it doesn't appear as though Nintendo has announced anything to address this yet.