I can easily see the next generation becoming much more heated as far as competition goes. Sony is bound to lose a good chunk of market share on the next console (atleast in the US), MS has a following now, a name attached to them, and people won't be scared they're just going to go away.
I think Sony's wanting to wait to release the PS3 until after the Xbox so as to not get as techologically overwhelmed like they were this gen. But I really think MS coming out first is going to cement them a nice chunk of market share and hurt Sony in the long run.
Nintendo will always have its following, and between the next console and the DS.. they're sure to keep a nice chunk of market share also.
So in summary.. I can easily see something like a 40% Sony, 35% MS, 25% Nintendo break down next generation and that could make one hell of a war.
But it's all bullshitting, who knows whats really going to happen.