Minimums:

PS3: 40M; first party software shines through + PS brand and PS2 users upgrading
360: 25M; lack of support for microsoft brand outside USA
Wii: 50M

Probables

PS3: 60M
360: 45M
Wii: 55M; massive decrease in sales after 2 years

Top Ends (in 2010 not 08)

PS3: 100M; after good late 07 and 08, 3rd parties turn back to Sony
360: 55M
Wii: 65M

Wii - 40 M, even if it dies next year its momentum should keep it going a bit longer.

360 - 30 M, just from lasting longer than the first Xbox even if it stays stagnant forever

PS3 - 20 M, pretty much only if everyone gives up on it

I'll throw my hat in the ring.

Wii: 70m
360: 40m
PS3: 25m

I think people are underestimating the 360.  It may never do well in Japan, but when it gets a price cut, 360s are going to move. 

360: 35 Million
Wii: 60 Million
PS3: 25 Million

That's aobut what Id say.

Wii 40

360 30

PS3 20

I'm saying these absolute WORST case scenarios

The PS3 refuses to drop it's price early enough and follows the GC track it's whole life : 20 million

The Xbox 360's sales decline continues, and for whatever reason Halo 3 doesn't pull it out of the spin: 30 million

For some reason, hidden to the logical mind, the casual consumer stops purchasing the console with the most games and the Wii's sales sit at what casuals it picks up over the next 2 years plus the few to the end of the generation: 50 million.

We are 20 estimates..

10 more and I'll do an average.

Wii - Almost certainly going to be the only console this gen to go over a hundred million, but at the very least going doing better than the NES. I'll say 70 million as my lowest estimate.

Xbox 360 - For now it's selling slightly better than the original, launched much earlier, and has way more exclusives. There's a good chance it'll sell extremely well after more big games arrive, prices drop, and the hardware issues are fixed, but assuming it doesn't pick up I'm estimating a minimum of 30 million units. I mean there's really no chance of it doing worse than the original Xbox anymore.

PS3 - It would seem only a few here agree with me on this, but I believe there is a good chance the PS3 will have a very short lifespan. It achieved notable launch numbers due to the amount of people who were ready to buy the next Playstation regardless of cost or games, but sales have slowed to completely abysmal levels. It still has a lot of exclusives that are too far in development to cancel, and probably under contract not to go multi-plat, but if those don't seriously boost sales then it's extremely likely the next wave of notable exclusives will skip it entirely. If the suits in charge of Sony decide the PS3 is an unrecoverable mess, then they might well cancel it before it hits even ten million units. That's still very unlikely, so I'll say a minimum of 20 million. Even that sounds a bit high for the worst case scenario, as that's what Nintendo got with their notably better (selling) first party software and a console with 1/3 the price.

PS3:
Low end: 18 million
Actual expectations: 26 million
High end: 35 million

That's right, 35 million is the absolute highest I can imagine PS3 selling. It will not reach a mass market price for at least 5 years, at which point the generation will be all but over. Its lineup of killer apps (GT, GTA, FF, MGS) is roughly comparable to the cream of N64s lineup, except that it relies on third parties and includes a non-exclusive. I can't see it doing less than 10 million in America, 3 million in Japan, and 5 million in Europe, though, and realistically I expect it to do 13/4/9.


360:
Low end: 24 million
Actual expectations: 32 million
High end: 40 million

With sales diminishing in America, things are looking down for 360. It can no longer be argued that a price cut will help the console compete with Wii, it needs a price cut just to get back to XBox levels. With the current sub-XBox level of sales, that leaves 360 at about XBox's total even with an extra 2 years on the market, due to the sales slowdown 2nd and 3rd place consoles experience late in the cycle. However, I expect Halo and GTA are going to provide a major boost for the console, and think the differences between Wii and 360 as opposed to PS2 and XB will help 360 maintain higher long term sales than previous 2nd place consoles, which would help the console move well beyond XBox.

Wii:
Low end: 80 million
Actual expectations: 150 million
High end: 40 gazillion??

Nintendo would really have to blow it to sell less than 100 million. (I think videogames have reached a point where the winning console is ALWAYS going to sell 100 million.) I see them selling over 30 million in Japan, over 60 million in America, and over 50 million in Europe/others--basically a large incremental audience growth in some ways similar to PSone. But if they screw over Europe completely, doubt their American success and focus on Japan too much, and then only reach PS2-levels in Japan instead of drafting off of DS, they could only reach maybe 80 million. There's always the possibility Wii is bigger than anyone can imagine, too, thus my 40 gazillion top end.

these are all worst case scenarios, unless of course we see another global market crash, then anything goes

360: 30 Mil, even if everything goes wrong and Sony winns again 30 Mil is the minimum,

PS3: 20 Mil, I know this sounds like a bash but considering it´s price and considering everything that (still) can go wrong goes wrong, all the High Dev games go to the 360 the PS3 would crash worse than the 360 with a lower price and the head start,

Wii: 50 Mil, again this sound like a fanboy statement but there just isn´t so much that can go wrong for Nintendo, even if this unrealistic fad argument catches on and everybody needs HD by next year and all the third parties leave and the new demographics lose interest, Nintendo still has it´s own stuff and will sell at leastl 2-3 times what the GC did,

I just now see that naznatips copied my ideas before I posted them, well great minds think alike

@ kber: if you think Sony can sell 70 Mil if everything and I mean EVERYTHING goes wrong what will they sell if everything goes right ??? 200 Mil, I know it can have a long livetime but 70 Mil is really high for a WORST case scenarion, 50 Mil, ok but even the N64 only soled 60something Mil. and it was far from beeing a worst case scenario, but thats ok, who knows maybe I just don´t give the Playstation brand the respect it deserves