PS3 - 30M
Wii - 80M
Xbox360 - 45M

Xbox 360 - 30 million
Being a lowest estimate we must assume the worst. If PS3 takes off with Blu-ray and drops the price into a market friendly range then this will have severe impact on 360's install base.

Wii - 50 million
With the momentum that it has with the general public, even if it faulters in this and starts to drop hype it will still be carried by the underpublic which are those thousands or millions of followers who buy a popular product on its way out. Especially with price drops. This is assuming that Wii becomes a fad though, which is a pretty big assumption.

PS3 - 17 million
Assuming that Blu-ray gets skipped altogether and digital downloads take off or Sony decides to stop making the PS3 due to low sales or cannot drop the price enough to make a difference or does something else stupid like bringing in something more expensive with more power or any combination of these.

Hmm, I believe this generation will be the longest so far (10 years or more) so my number is pretty high. 6-7 million a year... More than 120 million if everything goes right. I think PS3 will be cheaper than $300 for most of its life.

Worst case scenarios:

Wii - 45m.
Nintendo's momentum dies in 2008 as third party developers find that they are losing sales again to Nintendo's own offerings. The "waggle" and graphics become unappealing as more people move to HD sets.

360 - 33m.
I don't see how the 360 can possibly do worse than the original Xbox. Add in the extra year of life and the fact that it STILL hasn't seen a price drop and ~10m more than the original 'box is the worst I see it doing.

PS3- 21m.
Again, worst case. MGS and FF go to 360, price drops don't come quickly enough, Blu-Ray adoption rates stagnate.

More realistic but still pessimistic predictions:

Wii - 60m.
360 - 38m.
PS3 - 33m.

Maybe, but... that number wasn't worst case...  I am expecting a higer % of people with 2 or 3 consoles from this generation of current video game users. Also, expanded audiance range increasing average age of consumer, a much higher increase in sales to females, and more countries. A big boost from China by the end of the generation alone could really increase WW numbers.

Could we try to keep this thread for making predictions instead of clogging it up with arguing, please? 

four more predictions and i'll do an average.

OK, let's all try and be reasonable!

Alphabetically:

PS3: If we're talking minimums, I can see this system theoretically selling an average of 100K/week worldwide for the next 5 years. Obviously higher on holidays, but this is also taking into account the "last year" slump (assuming *roughly* a 6-year life), that would be 26 million, on top of the 3.42 they've already got. Since my estimations went to the conservative, I'll give a bump and say the minimum would be 35 million.

Wii: Wow, this is the hardest to predict a *minimum* for. Geez, we haven't even seen any sales trends that weren't directly affected by supply yet. I don't think it'll sell as well as the DS (a family may buy 2 or 3 handhelds, but probably will only buy one console), but I definitely see it doing better than PSP. So somewhere in-between. Those two have been out for almost a combined 5 years, and combining their current numbers, you get 66 million. I don't see Wii being cut before the 6 year mark, but my little calculation didn't take into account the fact that DS and PSP haven't had their last-year slump yet. let's call this the minimum: 60 million

X360: I've said it somewhere before, but we're going to find out in September if Microsoft is really going to expand their market. Honestly, I think there's still a stark possibility that they may grab *only* original X-box adopters, plus a couple million in change. Again, since we're talking minimums, I'll say 30 million

To help TheSource getting his 30 predictions (love your writings) I'll add mine and thereby have my first post:

Lowest total market (no killer machine like PS2 this gen): 130 M

Wii: 70 M
(first year incredible: next 2 years should see a lot of great (3rd party) games, after that maybe declining sales)

PS3: 35 M
(bad start, but great games coming, probably longest lifespan + Japanese support)

X360: 25 M (No Japanese support, struggeling over past months, same market as Xbox)

you may be right with your 10 prediction, but I find it very hard to tell by now, fact is that the loser(s) decide when the Next-Gen comes, if nobody feels like a loser it might take longer than any other generation before it, but the longer it takes the more likely it will be that somebodys sales got down, it´s allways been that way and even though this is a different Gen I don´t see a tie situation holding for very long, the question is will the first loser give up or put a new machine on the market, if and only if the PS3 can still fight a new machine for longer than two years it can have a 10 year livetime, my gut feeling sayes chances are slim to none for this to happen but I´ve been wrong before, or have I ???

@Parokki: sorry