Yes, the success has extended the life of how long the X360 will be manufactored, but not the time when next gen starts.
The Xbox 3 will launch in Nov 2011 and is the beginning of next gen.
The X360 will be manufactored until some time into 2013, and it's overlapping sales (relative to the Xbox 3 and the next gen) will be rufly 4 million in 2012 and rufly 2 million in 2013.