XBox 360 will continue to dominant over the PS3 in America only. Europe and Japan PS3 will outsell the 360 this year.
Natal may kickstart 360 sales and the 360 will continue to sell well in America and 360 sales will improve in Japan and Europe. With Natal's arrival and following in the Wiis footsteps, Microsoft will not have to release a new system for a while. No point launching a new system at the moment if you can only make a system slightly more powerful than the PS3 and with a heavy price tag.
The PS4 will be same time as the next XBox in 2012 or 2013. Next XBox to be launched in 2012 or 2013.
The 360 will see both Gears of War 3 and Mass effect 3 released before a new XBox is launched.