Let me begin by setting a couple parameters.

1) Don't say 3.42 million for PS3, don't say 8.17 million for Wii, and don't say 10.0 million for 360.

2) Assume at the very least that all three are continually sold through Dec 31, 2008 (no Dreamcast arguments).

3) Be realistic - Sony is going to end up selling 125 million PS2s in 7 years.  Microsoft sold 24 million Xboxs in about 5 years, Nintendo will end up at around 22 million GCs in about 6 years, Dreamcast sold 9 million in about 18 months.  For each console, the last year dragged down year sales averages significantly.

4) Last generation 180 million or so consoles were sold - the market is probably not going to shrink.

That said here is my take on the absolute lowest amount each console could realistically sell this generation:

Xbox 360 - 35 mllion

It is already at 10 million, it won't be killed as early as Xbox, it is selling about as well as Xbox at a higher price despite most Xbox sales coming at a cheaper price.

Nintendo Wii - 50 million

The launch was better worldwide than N64.  Sales in Japan in 6 months (June 10 was the 6 month marker) are already 1/2 of lifetime N64 sales - a console which launched so strongly that 1/2 of it's sales in Japan happened by December 1997.  Even if Wii declined like N64 in Japan, it would likely mean sales of 10 million+ since Wii will be at roughly 5 million by the end of 2007 in Japan.  I expect Wii to do at least N64 numbers in the USA, and I think it will be Nintendo's most successful console in Europe, where sales are comparable to Japan on a weekly basis.

Playstation 3 - 40 million

Sony, almost no matter what, is going to do at least 7 million in Japan, and at least 20 million in Europe and the Americas on strength of brand alone.  However, the huge userbase was based on people with no brand loyalty who simply thought PS2 was the best product (most games).  In the worst case scenario for Sony, it turns out that 2/3 of their userbase is this kind of gamer, and abandons the Playstation brand.  With Sony doing almost Gamecube numbers at $600, I would expect sales to increase fairly dramatically below the $400 barrier, meaning that the best years of the console will come later (2009/2010) as long as 3rd parties continue to support it.  

that sounds about right, although i would most likely place the xbox 360 also at 40 million, maybe 45, it has a way better momentum than the normal xbox had and it got a lot of good games coming up, such as GTA, that usually sells consoles

Interesting post. I would say:

PS3 - 70 million

X360 - 45 million (the biggest winner of this gen)

Wii - 35 million

Good to see that TheSource is another member of this community who understands time factor regards sale of PS3.

I think the low end of the ps3 would be about 30 million. Because a negative momentum of developers abanding it could take place and kill sales till it hits 300$ or so, then it's momentum would be at xbox levels but with better support in japan.
If HD-DVD takes over as leader, then it'd really die fast. Europe&America wouldnt hit past 15M on brand alone at that point. When they stop buying they stop buying Fast over there. I'd give this about a 5% chance of happening.

360 - prob 40 million, on pure momentum at this point.

Wii, yea 50 mill is prob the lowest possible on how much word of mouth is helping it.

110 million is lowest reasonable estimate huh?

PS3 - 30 million
360 - 35 million
Wii - 50 million

These are obviously all minimums.

As I see it, the Minimum realistic numbers for the PS3 and XBox 360 sitting at 25 Million units each and the Wii I see at about 40 Million.

The fact is that the XBox 360 has not sold dramatically faster than the original XBox and the PS3 is currently selling slower than the Gamecube did so they both could fall into a similar sales range. The Wii (on the other hand) will likely break 20 Million this year and if things go poorly it could start selling at a similar level to the Gamecube/XBox for the rest of its life.

If we can get to about 30 opinions on this, I'll average it out.  I think all of us are a little biased, but an average opinion should be much more reflective of market trends.

We have 6 so far.

realistically

30 million for 360-seems to be slowing down, shooter market may be reaching saturation

25 million for PS3-Selling under the GCN, far away from market sweet point price and facing cheaper alternatives, time is not on its side with that $200 price point very far away

40 million for Wii-very small possibility of market drying up, but not likely, this is the least likely low end prediction to occur

My Worst Case scenario estimates are...

PS3 Sells like DC - 10 million
360 Sells like Xbox - 25 million
Wii Sells like N64 - 33 million

My more reasonable "pessimistic" estimates are...

PS3 Sells like GC - 25 million
360 Sells like N64 - 33 million
Wii Sells like SNES - 43 million

My lukewarm estimates are...

PS3 Sells like N64 - 33 million
360 Sells like Genesis but better - 40 million
Wii Sells like NES - 63 million

This is based on short to mid term though, things will definitely change.