My prediction: The pie will grow. Currently there are about 120M current gen consoles sold, I would think 140M-150M will be attainable next gen.
PS3 will win overall world wide. It will be a an awesome piece of HW with not only excellent gaming but great home theater support. Powerful and practical in approach the PS3 will remain about gaming. Their pie % will shrink a little, but they will still 70-80M consoles in the same time frame that they did the last 2 gens. It is tough being first because their is only way but down. Sony will do a good job defending their throne.
Xbox 360 will be neck and neck with the PS3 in North America and they will make it closer in Europe. MS will make a token showing in Japan but will continue to be a distant third. MS's plan of leveraging the Xbox game platform and the Windows game platform begins to take life with Longhorn and Live becomes a driving force in online gaming. They will slightly increase their total marketshare % but will substantially increase their total sales.
Nintendo will appeal to their core demographic and fans and catch some new gamers with their new ideas. But the Revolution is more practical than evolutionary (it can play GCN games, so it cannot be too different). They will struggle more than ever before with 3rd parties and will ultimately lose some market share %, but will continue to be in the 20M-25M total sales. The big news will be the GameBoy 2 which will build on Nintendo's success in the handheld area.
I think a lot of factors will result in a bigger market. I think Nintendo will seriously try to branch out to new gamers, the PS3 BR drive will appeal to the home theater market, the PSP will begin to suck even more mainstream people into the home console market, and MS will offer convergance features for email/browsing and the like, Sony will continue to experiment with EyeToy type devices, and so forth. I also think each of the companies will try to reach new gamers (e.g. one of the Xbox heads stated today the need for more casual games). I think we will see companies begin to seriously target girls, moms (one of the biggest gaming demographics), and casual gamers more and more.
Of course this is total conjecture based on the direction the companies are going and what they have, or have not, said about their next gen offerings. So I reserve the right to be totally wrong.