The only reason MS started this generation early is because they were unable to gain ground on the PS2 and they were stuck with a console that was not planned properly for cost reduction. (Required HD and bad deals on hardware, the GeForce4 GPU in particular)
By starting this gen first they turned the table and now Sony has to play catch-up instead. Unless 360 hw and software plummet they have no reason to launch the next platform. Considering that the 360 has been the best selling console platform for software during 2007 (in the US and not even counting Halo3) I don't think they are in a hurry to go next-gen. They need to make some money back on this gen, at least some of it, before the next platform. Personally I don't think we're going to hear anything about it before 2010.
Regarding 'Xenon', this was the code name for the console itself before it was named Xbox360.