I think they will launch in 2013/2014 with the Next Xbox. With both selling well and (probably) making good profits there is no reason to jump into a new system. (Even with PC's graphics being a whole gen better, customers are still buying consoles.)
Now, if the WiiU starts dominating, they might move things up!
PS Just want to remind everyone that many at MS thought the PS3 was going to launch in 2005, which is why they brought out the X360 at that time. It makes since PS1 1995, PS2 2000, so PS3 might have been 2005. They wanted to stay current and seemed to have rushed and obviously had a problem. I think they were trying to stay current not so much get a 'jump' on the competition. Just turns out that they did.