Nintendo will ride the Wii out for as long as possible, will keep stuff ongoing with different hardware colors, and more hardware SKU's, like one that provides 720p/1080p/HDMI output, another one with more internal storage space, etc. I wouldn't expect a new Nintendo system for a while, and even then, I can't begin to imagine what it would be like... if it's gonna be an incremental advancement to the Wii's control tech, or something entirely different.
Sony is gonna try and recoup their PS3 losses as much as possible, and hold out their system till as long as possible. And even then, I expect its followup will use Blu-Ray and Cell as well, being backwards compatible with the PS3. The R&D cost will be much lower (by this time, Cell manufacturing will have hit its stride and Blu-Ray's costs will have come way down) but the system will still be pretty graphically capable, and waggle will be a lock for the new system.
I'd say MS is gonna be the first to the market, once again, with a next-next-gen system. Especially if the PS3 secures a worldwide lead over the 360. The system will probably feature Blu-Ray and an obvious generational leap over its predecessor, and even more focus on Live/downloadable gaming. Hopefully, this time they won't repeat their past mistakes and kill off support for their previous system completely.
I'm more so interested in what Nintendo is planning to do with the followup to the most successful game platform of all time, the DS.