I think it all depends on how well 2008 goes for Microsoft ...
If sales pick up Microsoft will (probably) plan to launch the next XBox in late 2011/early 2012 and they will (potentially) break 50 Million XBox 360 systems sold by the end of the generation.
If sales remain about the same as they have been Microsoft will launch the next XBox in late 2010/early 2011 and the XBox 360 will fall short of 50 Million sold.
If sales of the XBox 360 begin to drop off Microsoft will launch the next XBox early in 2010 and fall far short of 50 Million XBox 360 systems sold.