I seriously doubt the PS3 will over take the 360 in America. Atleast anytime foreseeable in the next few years
... unless Microsoft pulls the plug on the 360, releases a 720 and stops making 360s.
... or Sony somehow can afford to sell the PS3 at a even lower price.
If somehow things were to get worst I could easily see MS lowering the price of the arcade and elite even more, even with a hardware revision. It's Microsoft for God's sake, they lost billions on the original XBox just to get a foot in the door in the console war.
I think It's pretty apparent that MS wants to do well in America. Looking at the majority of their exclusives and best selling titles they come from US software makers to. Their audience is very western centric so losing to Sony in the US would hurt quite a bit.