Well, that's an interesting thought. I don't think too many people in the past, would have considered the idea of the PS3 not reaching 25% realistic, but it does look like it's a definite possibility now. That is, if both Ninty and MS are aggressive enough. With all of Sony's loses, they might be too budget constrained to be able to do a whole lot about it. Either way, I have to say, coming from the previous gen, this is already a major victory for both Ninty and MS.
Rounding numbers a bit...
Nintendo's at 35 million, and making 2.5 million Wii's per month. It is easy to see how they can be at 60 million next year (12 months x 2.5 = gain of 30 million).
The 360 is at 22.2 million, the PS3 almost 16.5. So combined they're at nearly 39 million. In order to avoid falling under 50%, they would then need to sell, combined, a minimum of 21 million consoles.
In the last year, the 360 has sold just under 10 million, the PS3 just under 11. That keeps them on pace with the Wii, however, it also means the PS3 would only account for 28 million of 120 million consoles - or 23% (less if Nintendo sells all 30 million consoles, and/or increases production again).
Of course it is likely that the PS3 sales will increase somewhat due to a price cut and big name games, but there is no reason to believe those gains will be substantially out of proportion to increases on the 360 and Wii (which could cut price tomorrow and still make Nintendo a mint).