I think for christmas 2007 the PS3 will show important sales victories in Europe, Australia, Japan (duh!) and other asian countries. For North America I expect sales to go neck to neck, mainly due to the release of Halo 3.
I expect the PS3 to surpass the XBox 360 total hardware sales somewhere late in 2008 and finally when a slimline version appears and the PS2 is retired to dominate significantly over the XBox 360.