Right now Wii ticks a couple of boxes very nicely.
1) Fun and innovative and def choice for family/casual
2) loyal fanbase around core franchises (Mario, etc)
3) Zeigtgeist - its got it and you can't plan for it
4) price point and designed for current TVs
On the other hand I definately see a big difference this generation and I feel that going forward we're not going to see Wii totally take over and 360/PS3 simply fail. The reason - the hardcore and HD gamers.
Wii clearly isn't going to become the home of multi-million selling FPS / MP games anytime soon. There is a sizeable market that is going to go with 360 / PS3 right through this generation and its also a very lucrative market too.
Also, the whole games market itself is growing massively.
I know many homes with a teenager with a 360 or PS3 and a Wii for the younger kids and family fun.
I know many Nintendo fans with only a Wii.
I know many HD gamers with either 360 or PS3 or both.
The differences between Wii and 360/PS3 prevent direct crossover. If you want games like MGS or Gears of War they aren't coming to a Wii no matter how big its marketshare gets.
On the other hand neither 360 nor PS3 will ever have the core Mario games nor is it likely they will capture the Zeitgeist the way the Wii has. I feel the PS1/PS2 torch as the ubiqutous console has currently been passed to the Wii - but the Wii's specs mean that those who want GT in HD will still be getting a PS3, and those who want GoW2 will be getting a 360.
So all three are going to sell well enough and get enough games support for the foreseeable future.
The only real region that truly seems to have settled on one console is Japan, but recent evidence with PSP suggests that if they decide to start getting into HD based gaming (i.e. HD grahics) then the PS3 will likely pick up there later in this cycle (FF and MGS may of course accelerate this) IMO (360 I think just has no chance to be the HD console of choice in Japan should that opportunity ever arise).
Try to be reasonable... its easier than you think...