The 360 is currently in the part to where it is having a midlife crisis. That means the 360 is probably a woman, and needs a makeover.
But in all truth, given what has happened, it is about 50-55% of the way through its lifecycle (to where it is a relevant console).
NATAL proves that Microsoft is serious about keeping the 360 alive for at least 2 years after NATAL launches. There would be no reason to keep developing on the 360 with new R&D if they were going to merely kill it in a year or so.
At any rate, the 360's sales are slumping because its in a doldrum: it didn't have a ton of AAA software to round out the year (which all got delayed to Q1 and Q2 of next year). Prices were still similar in terms of the lowest price point, so there wasn't a new market that could get the system at a cheaper price.
Throw in the fact that its still (externally) the same system we knew in 2005, and it really does need a re-branding and renewal effort. The X360 is a great system, but really needs a slim version. I think that NATAL will involve a new slim 360 to drive adoption of NATAL, as well as push for people that haven't purchased a 360 to buy the new-looking system, ala the PS3Slim.
Back from the dead, I'm afraid.