I'd say of the 3 current system makers, i'd say Nintendo since MS and Sony will soon be adding on their new motion-games on their current systems.

I could see the Wii continuing stellar sales a couple of years into the lifespan of their next console, so I don't think that will make Nintendo wait any longer to come out with whatever they have cooked up.

Most definitely not Nintendo.
I would say Sony or Microsoft. They have the lowest demand in this generation, and historically low demand means short momentum, big demand means bigger momentum.
If I have to chose one, I go with Sony, I expect Microsoft will try to take seriously Natal, and even if it would fail, they would wait until its outcome in late 2010, before starting to plan the next generation.

Releasing first doesn't mean very much, it didn't save XB 360 from Wii and from its own faults, PS3 too wasn't able to fill the gap for its own faults more than for the gap itself, actually most people and even MS expected it to narrow the gap faster. Enthusiast early adopters are always the vast majority of buyers during the first months, whenever the release happens (big, big error for Sony having shortages at launch). Releasing first gives an initial advantage, but at leas two big drawbacks: higher costs and in the HD "muscle" competition, the opportunity for competitors to release slightly more powerful products at slightly lower price, if willing to do it (again another thing Sony wasn't able to exploit for excessively ambitious project, Nintendo OTOH didn't because it had a radically different plan, completely ignore HD and whichever muscles aren't players' own ).
The only reason for Nintendo to release a "Wii 1.5 HD" would be if Full HD TV sets are adopted by the vast majority of families before the previously expected end of this gen, but a smooth enough upscaling with AA, to avoid noticeable jagging on bigger HD screens, could be enough, given the scarce attention to games resolution demonstrated by Wii gamers up until now.

I think Sony will make their console second. Microsoft or Nintendo will probably be first.

Probably Microsoft.

It will be a new gen contender, funded by the mexican government.
The Mex Box, it will have President Evil as an exclusive and GTA Mexico City, Power Ranchers and for the kids Marley the rastafari dinasour game. and will be the cheapest of the consoles.
It will sell in every single super market.