My prediction for the next generation is pretty different from most opinions here. Toward the later quater of O7, the 360 will gain ground in Japan, but not much. Major titles such as GTA, Mass Effect, Devil May Cry 4, Virtua Fighter 5, Blue Dragon(In US and EU), Lost Odessy, and of course Halo 3 will ensure that the 360 dominates the year 07. The Wii will continue it's fast track and will come within 1-2 million by end of 07 This is overall, not in any specific teritory. The PS3 having only 3 major titles coming out this year to my knowledge: Tekken 6(I think), FFXIII, and MGS4 will continute to lag behind it's competitors.
2008
The 360 my hit it's first actual major title drought, but consumers will have been so Wow by the late 07 releases that it's momentum may continue to grow or atleast sustain itself. Until the major titles start coming up again. During the slow period MS may make a price drop of $50-$100 putting the Core system below the price of the Wii, but this won't happen until MS has squeezed every bit of cash they could from their stellar 07 showing.
-Sales Estimate(end of 07): $17.18million
-Sales Estimate(end of 08): $22.24million
The Wii may face some difficult times in 08. After the 360's 07 showing, graphics may be pushed back into the for front. If this happens the Wii is screwed. While there will be some stellar uses of the Wiimote, many companies will fail to utilize it effectly. This is more damaging to the Wii than to most other platforms because the Wii emphasizes gameplay innovation and lacks the pretty bells and whistles to make up for what it lacks gameplay wise. While this won't happen too much, roughly 25%-33% of the games released will have this problem. Unless the Wii can wow with more than Wii Sports and Twighlight Princess, it will begin to fall into obscurity at worst becoming a party system that you only need a few games for.
-Sales Estimate(End of 07): $13.80million
-Sales Estimate(End of 08): $17.76million
This will be a very important year for the PS3. After being hammered by the 360 and Wii, this will be the year that will make or break it. They will continue to lose exclusives to the 360 due to it's larger install base and similar capabilities. Fortunately for the PS3 HD televisions are getting cheaper and cheaper. Due to it's BR-DVD capability the Blue-Ray format will win the format war which will help the PS3's sales, but the software will still struggle to move as it will be seen more as a cheap BR-DVD Player than a game system. If the PS3 is to thrive, it will have to show the "true" power of it's Cell Processor and Graphics card, otherwise in the gaming world it will always be seen as an expensive Xbox 360 due to the amount of ports the two systems will share. My personal opinion is that the PS3 will not come out with anything so stunning as to set it apart as vastly superior to the 360.
-Sales Estimate(End of 07): $6.80million
-Sales Estimate(End of 08): $12.41million
2009 and Beyond
The 360 will enjoy great success everywhere but Japan where it will still struggle. Many major 3rd party games will be ported over to the 360 like Tekken 6 and MSG4. More likely than not I don't see Square-Enix bringing FFXII to the 360. But the price drops will continue to keep it competitive with the Wii in terms of pricing. The 360 will become what the PS2 was last generation as the preferred platform for games. It won't sell as many consoles but it will sell enough to cement it's lead over the PS3.
The Wii have completely shown it's weakness and will be supported by Nintendo 1st party games. Nintendo will play it smart though. Considering the power differences between the PS3, the 360, and the Wii, I don't think Nintendo spend as much time as the other companies working on the Wii. Instead they've only spent half the time working on the Wii and the other half making a "Next Gen Console"(Meaning the Generation after the Wii/360/PS3) This will place them in perfect position to launch first probably a year before MS and about 2 years before Sony.
The PS3 will have the second largest user base, but the lowest attach rate of any system in recent memory. Sony will cut the price of the PS3, but at this point even a $100 price cut won't help. They will have gained solid control of the next gen format. Personally that's what I think they where going for all along. The PS3 shop will surpass Xbox Live in sales because Sony has so many entertainment avenues that MS does not.