First look at these two graphs comparing the Wii, PS2, and DS in Americas and Japan. As you all already know I had to leave out others since no data is available for PS2.

First Americas:
http://vgchartz.com/hwcomps.php?cons1=Wii&reg1=America&cons2=PS2&reg2=America&cons3=DS&reg3=America&align=1

Notice that the DS falls off quickly where as the Wii is still selling at a higher rate than the PS2 thus far. However, it is very close.

Second Japan:
http://vgchartz.com/hwcomps.php?cons1=Wii&reg1=Japan&cons2=PS2&reg2=Japan&cons3=DS&reg3=Japan&align=1

Here DS is showing its clear dominance, and again the Wii is close to the PS2, trailing this time, but close and on the way to pass PS2 well before the DS did.

Notice here the Wii is out selling the DS by a fair margin.  Given this and assuming the sales rate remains the same, I beleive the Wii has a very strong chance of becoming the WW leader in hardware sales.

 That's pretty much my thoughts as well.

Wii will never pass PS2 numbers.

Imaging how high sales are going to go up for Wii once production goes up and there is a more robust library of games.

I think that it will not surpass PS2 in US, but will in Japan. World Wide it definitely will as the "other" expands - Korea, China, Eastern Europe, and as a tool for Putin to say "Hey I am not so bad. Y'all have bread and circu... I mean Wii."

Wii won't catch the PS2.
I doubt the longevity of the Wii compared to the PS2, and ultimately I don't think it will reach those numbers.

Unfortunately, I think both camps make reasonable points. I'd love to give an "absolutely not" or "absolutely, yes," but I don't think either of those is reasonable at this point in time.

I hope those of you who think the Wii will pass the PS2 can agree that the sales could slow down at some point in the future. I'm not saying it won't sell well -- you all know my opinions on that -- but even if it ended up selling 80 percent as well as it does now, and lasted for 5 more years, it still would not catch the PS2. So saying "it won't catch the PS2" isn't even saying it won't win this generation; it just may not end up dominating the market as much as the PS2 did, and I think that's reasonable.

However, I also hope that the nay-sayers will agree that there is some possibility that the Wii will surpass the PS2. Personally, I think it depends on how many people actually join in -- and enjoy -- this casual revolution and sudden inclusion of more adult gamers. If it's 5 million people, that's great, and will do a lot for the Wii. However, it's going to need ot be 20, 30, 40 million new gamers to really give the PS2 a run for its money, I suspect, and we don't know yet just how many new gamers the Wii will bring in yet. I hope you'll agree that Wii Fit et. al has a chance of really including a lot of people in gaming that had no interest in it before, and if that happens, the Wii has a shot. Furthermore, as other posters have already stated, the Wii is on track to surpass the PS2 at this time. It's early, but that's a very good start. 

Personally, I give the Wii a 50-50 shot of surpassing the PS2. That's not just me being lazy with my fractions; I really feel the odds are juuuuust about split down the middle right now. Regardless, again -- no matter how boring an answer it is to say "maybe," I hope both sides will admit that "maybe" is the right answer right now.

I think hitting the PS2 figures will be much harder for the Wii.

Mostly due to Nintendo's outstanding quality track record. The PS2 was terrible in design for quality. Having purchased 3 PS2's I have to say the "mighty" numbers for the PS2 are a bit inflated.

Wii has a shot of actually expanding the market enough to reach those kinda numbers at its zenith. Although I do feel that most the the converts will not be buying a Wii on their own, they will be presents from their kids and such. Software sales is where Nintendo will be reaping the most money off the new casual converts.

I think if Nintendo is able to pull off the mass conversion of casual people to console gaming, Wii 2 has the best shot at setting records.

Bodhesatva, brilliant post.

Quickdraw, it really depends on more than what is currently visible. Will 3rd parties give more REAL support? Will the casual market really change things? Will the PS3 or Xbox360 reach a truly attainable price point any time soon?

I think that the casuals haven't even started backing the Wii yet, because they are still too hard to find. A casual gamer isn't going to wait around for a video game, they will get one when it is more readily available (and cheaper). The stats are showing that 90% of buyers owned a console last generation. I think it will mimick the DS pretty closely, when it drops to $199 (in 2012 :P ) and there are penty in stock (also in 2012 :P ) thats when we will see if casual gamers are really gonna make a difference.

Its too early to tell, but my first guess would be no. Those are some big shoes to fill. The Wii's hardware is less comparable to its competitors than the PS2's was to its competitors as well. That may not be important considering the market the Wii is aimed at, but it is a factor nonetheless.

Honestly, I don't see where all these claims about the PS2 being a faulty piece of hardware come from. Most people complain about "disc read error" which is from, duh, a dirty disc drive. All I did was clean mine out and it worked like it was brand new. I have two PS2's, one 6 years old and another 5 both of which have had no problems except the disc drive needing to be cleaned. Its not like this doesn't happen with other hardware, like DVD players and CD players. Dust and magnetization interfere with reading the disc, but using a product that cleans it or manually doing it completely solves the problem.

Although you may be right in saying that akuma (I skipped the entire last gen, so i don't know), other people would see "error" and get a replacement.. even if it wasn't really the fault of the hardware