1. Will they? Yes.
WHEN will they? That's the more interesting question. Like the SNES, Nintendo isn't going to move the Wii until they are disrupted by another console. Sony & MS are also in interesting positions.
Sony: Sony is working hard to make the PS3 profitable, and that might take a while. They also need to reevaluate why they are in the console business. They thought consoles would be the one-stop media player paradise in everyone's living room, but the Wii has easily taken that spot. They could try to clone the Wii but the movie & music branches from Sony would disagree with it. Expect another console next gen but...it'll be interesting to watch which way Sony goes.
Microsoft: MS entered the market to prevent Sony from becoming the one-stop media player. MS burned a lot of money doing it, and Sony has been sucessfully halted. MS also wants to integrate media playback into its console (through Windows instead of BluRay though) so going in the Wii's direction is also against their philosophy. Sony is making another console, so MS will make one also. If Sony were to drop out though, MS would lose a lot of motive to make a new console instead of pushing the X360's features.
2. Newcomers?
Entering the console market is very expensive (see Microsoft.) With such fierce competition, you simply MUST disrupt games to your values (like Nintendo did with the Wii) or you will be stomped by the incumbents. Very few companies have the money and the creative design necessary to launch a console. Apple, maybe?
There is no such thing as a console war. This is the first step to game design.
