GT5 will never be a 10M seller. The reason is simple:
GT4 got to 10M on the dominant platform of its days (the PS2) when the PS2 what its its peak momentum. With 100M PS2 units WW you need only 10% penetration to get to 10M games sold.
With the PS3 10% will buy you less than 1.7M units. If it is widely successful it will get 4M LTD if it will have long legs.
But 10M? - nuts.
Prediction made on 11/1/2008:
Q4 2008: 27M xbox LTD, 20M PS3 LTD . 2009 sales: 11M xbox, 9M PS3