It will only reach the 10 million mark if Sony bundles it with every PS3 until the end of this generation coupled with it selling like hotcakes these holidays. Too much time has lapsed between GT4 and GT5 leaving other games such as God of War, Uncharted, Metal Gear Solid and others to define the "Playstation experience" absent Gran Turismo.
I have predicted 425,000 in it's opening week and 4 million lifetime (I assume it will be bundled to give it the legs it otherwise would not have standalone retail).
As of the latest preorder charts, GT5 is at 260,000 total. No way it breaks the 1 million mark it's first weekend unless it magically adds another 100,000-200,000 preorder between now and Black Friday.