6 mill isn't doable if the data we have is correct "First week sales are expected to hit around 2.4 - 2.5 million units" (feel free to feed me MAJOR crow if I'm wrong)
Week 1 - 2.5 mill - Launching WW, Black Friday.
Week 2 - 400k?
If you look at Halo 3, Reach, MW2 360, Black ops ect, all with massive openings around 4/5 mill, all did around 600k - 800k 2nd week. Now you could say, they had extra days in the market VS GT5, but they didn't launch during BF week, with a huge Japan opening. Which brings me to my 2nd point, sales in Japan drop off VERY hard and fast for the vast majority of games there. So GT5 is going to see a very harsh drop off in week 2.
Beyond that I can't tell how the game will fare over xmas, those figures I posted were rough. But this is mainly in response to people saying 6 mill ...