Some people seem to forget thyat there is two part to "___ domination starts".
This was first used by PS3 proponents to mean that at that point (Gundam Musou, motorstorm, 2008...) the PS3 would have left the path where it sold less than its competitors and would begin on the path that would eventually lead it to domination (i.e. its momentum changes and start moving in a direction where it would eventually overcome the 360's and wii's momentum). For that to happen it doesn't need to sell 10 millions in a month to catch up with Wii or even to sell more than the Wii but it would need to improve its sales week after week until it eventually did the latter and for a long enough period for it to do the former.
Of course, with its continuous failure to do so and the numerous pushing back of it to later game release/price drop/dates it was taken over by Wii proponents as a sarcastic term for when Ps3 proponents tout Sony successes a little bit too early (like November Japanese sales). PS3 porponents, like any good doomsday prophet are now being more careful to foresee PS3 domination in a future that is far enough that most will not remember their post by then.
Now, as to who is dominating this market I would have to say that the 360 started dominating it by default of not having any competitors and its domination had its golden age around Xmas 2006 as the other two were supply constrained.
However, since the beginning of 2007 its domination has been waning while the Wii started catching up; thus, using the expression coined by PS3 fans the Wii's domination started January 2007. It wasn't complete by then of course but that month marks the turning point where the momentum of the wii became higher than that of the 360 so that the domination would eventually pass form the 360 to the Wii.
At the time the Wii and PS3 were both dominating over the 360 in Japan but not over each other yet. Since then the wii went on to dominate Japan and thanks to that to take the lead worldwide last summer and is now almost far enough ahead to be considered dominating worldwide IMO (which would be 5-10% total marketshare more than the runner up). It took until close to the holiday for it to take the lead in Europe and should dominate it in the next few months (depending on whether it goes back to being available in Europe after Xmas or stays almost sold out for a few months and depending on whether Wii Fit rises sales significantly its domination here might be as early as Q1 CY08 or as late as Q3 CY08 but unless the "Wii Fad" theory miraculously turns out to be true it will be dominating there by Xmas 08).
As for the US, given that it was sold out all year long and that Wii Fit hits next year, when it overtakes the 360 depends on how much of its production Nintendo decides to allocate to it but I doubt that it will dominate there until 2009 as it is the 360's strongest market and I believe that MS will seriously lower the price when jasper arrives, which will improve its sales and delay Nintendo's overtaking/domination there.
Even though the Wii's won't be dominating each individual market but Xmas 08 I expect it to be dominating globally in the first half of 2008 (i.e. Wii 45%; 360: 35% and PS3:20% give or take 1-2%) and to have 50% domination by the first half of 2009 at the latest (second half of 2008 at the earliest but possibly losing it during Xmas 08 if that is the case). The latter is unlikely as it would need the PS3 and 360 sales to considerably slow down, at which point they would cut the price to get them back up as their price is still very high (like Sony did this year, twice).
Anyway, if anyone care to put it in their sig, my prediction is that by the end of June 2009 at the latest the Wii will have more than 50% marketshare and will not lose that majority this generation.
"I do not suffer from insanity, I enjoy every minute of it"
