Xbox 360 - 20 million
Microsoft has still to show that they can sell their new box to different audiences than the ones that bought the original Xbox. The existing users will be pleased with the upcoming lineup, but if you want to sell to the casual demographics you need a price point at 200$ or lower combined with a varied lineup that has games for nearly every taste. If they don't show some games at E3 this year, that are different to the usual stuff that gets released for the 360, Microsoft won't reach out to the casuals ever with the 360. The 360 is selling at about the same rate as the original Xbox, it will likely end up with LTD of 20-30m.
Playstation 3 - 10 million
Sony is facing the problem that their console is pretty much the same as a 360, just with higher price tag and less games. They are already losing a lot of 3rd party support, multiplatform helps the 360 definitely more than the PS3, because the price difference and size of library remains the same. Japan doesn't seem to want to support any of the HD consoles, the 360 is as good as dead there, the PS3 is mostly sold to people who use it as a Bluray player, weekly hardware vs. software for PS3 shows that, at least for now. The PS3 is selling worse than the Gamecube, has a higher price tag and lacks the proven 1st party games the GC had. The way it looks, Sony has to entirely rely on 1st party games this generation, since most 3rd parties go multiplatform. It's realistic that the PS3 ends up with LTD numbers below the GC, 10-20m.
Wii - 110 milllion
The Wii is the fastest selling system ever, even outpacing the PS2 and DS. 3rd parties are shifting more resources to the Wii, which results in a varied (not only big in numbers) game library and that's the key to become a winning console. New innovative controls and lowest price tag of the three competitors are another big factor for the Wii's success. Numerous reports have shown that the Wii is selling also to people who have never had a console before, so why should it be impossible for the Wii to not reach at least PS2 numbers at the end of it's life? LTD 110-150m.