When talking to Joystiq Microsoft's Robbie Bach had the following to say regarding when we would get to stop calling it "Natal"? (Remember Project Natal is merely a codename)
"When Nintendo came out with the name 'Wii,' people sort of said 'Oh gosh, that's kind of a goofy, weird name. I haven't heard a comment about it being a goofy name since the week after they announced the name. And suddenly, people just called it the 'Wii' and moved on. And, specifically, they've moved on to buying them en masse."
Bach also noted the mainstream consumer has an attention span of roughly 3-6 months so don't expect Microsoft to come out with the official name quite yet.