IMO, in the US and Europe, it seems like the Wii has a long way ahead until it'd start slowing down.
Mainly because the X360 and PS3 top-tier games aren't out yet. Halo3 is just the first in a huge slew of games, that could (emphasis on could) steal Wii thunder.
You basically have:
Super Mario Galaxy
Super Smash Brothers Brawl
vs.
Halo 3
Mass Effect
&
Metal Gear Solid 4
Final Fantasy XIII
+ Non-Wii Multiports:
Grand Theft Auto 4
Call of Duty 4
Rock Band
Now, the question is: can SMG and SSBB continue the Wii trend of uber-sellers, and allow Nintendo to re-capture the inititive in Japan, and restart 70k/wk sales, and hit insane holiday numbers?
I know some argue that "oh, the Wii is just down like every other system is down". True, but only to a point. The Playstation 3 is nowhere near it's lowest, despite few good games out - Bladestorm and HSG5 are the only 2 major games selling right now. So why exactly has the Wii gone from selling 7:1 to just 2:1? If the market was totally in recession, why isn't the ratio keeping in lock? For the PSP vs. DS, it was handily going in the DS's favor until FF:CC released on PSP, creating an anomaly.
As I've been saying for the whole year, I still see the Wii sales cresting in the US and Europe next year, slowly losing marketshare to the PS3 and X360. Remember, there was a time for 3-4 months that Wii sales were beating X360 and PS3 combined rather easily. That's not quite the case anymore. If that trend continued, MS will regain the US per-month lead, and will stay there, as the X360 lineup seems to be more favorable in the US, and the PS3 lineup, in Europe.
IMO, I still see the race being that Sony gets Europe barely, MS gets the US by a decent margin, and the Wii still manages a good lead in Japan. But we'll see how 2008 goes with the influx of Wii software.