As we all know, Halo and the 360 are dependent on each other. One cannot thrive without the other.
Halo 3 was just released, and achieved enormous, unparalleled first day sales. But yet, how did the 360 sell around Halo time? Twice as well, yes. But now it's died down again, and still, 360 lags behind the Wii overall.
So my point is: The 360's big seller has come and gone, and the worldwide sales picture has barely changed. Not only that, but the 360 is 2 years into its lifecycle. Microsoft wants a 4 year cycle, so it's hard to believe the 360 will get much higher than 20m sold in its entire lifespan. When PS3 gets hot in 2008 it will feel fresh. At this same time, the 360 will feel 3 years old.
Now you might be saying: 360 consistently has the best selling version of multiplatform games. And thus, 3rd parties don't care about the userbase, as long as loads of people are buying their games. Those statements are true, but the PS3 is upcoming.
We all know PlayStation is the top brand name. We all know the price cuts have already started and will continue. We all know that development kits will continue for the PS3, allowing more games to be made, quickly for the PS3. I've also heard a lot of people considering the PS3 for this Christmas due to the new UK price cut.
I can't believe I'm saying this, but PS3 seems very likely to win now.
The only thing that can halt the thing is a lack of games, so you're going to have to assume the dev tools improve drastically.