This is due manufacturing problems with it's processor. As the Xbox 360 makes use of an Sun microsystems PowerPC nick, the following-age bracket Xbox is rumored to be utilizing a chip by AMD that needs to be arriving from the production outlines within December. Apparently the chips are not passing screening which could mean the delay.
Ideally any problems are solved quickly so they can unveil this at E3 the coming year, the industry small just one year. Take it with a touch of suspicion, however keep the fingers crossed.