ok, time to throw in my guess (haven't read the whole thread).
GT has traditionally been one of the best selling games on the PS brand. Sorta like Mario Bros. on Nintendo systems, so of course it will do well. However, the PS3 hasn't been moving nearly as much software as the PS2 did, nor does it seem to support 10m sellers, so I think there is no way it beats GT3 unless it becomes the next Wii Fit and every NASCAR fan has to have a copy.
Looking at past trends, GT (series) has sold about 80% of EMEAA in NA, and about 36% of EMEAA in Japan. Trends show that NA got dissinterested (in comparison with EMEAA) in GT with GT4 and the ratio is now closer to 50% of EMEAA. Japan seems to have lost and then regained some interest in GT (in comparison with EMEAA).
So, I'll give GT5 4.5m in EMEAA (best selling game in EMEAA by 700k) and then 50% of that in NA, and 30% of that in Japan. So,
EMEAA = 4.5m, NA = 2.25m, JPN = 1.35m
Total = 8.1m
5th place overall and the #2 game on the PS3 just behind Call of Duty: Modern Warfare 2. It would be the best selling in EMEAA, 2nd best in Japan only behind FF, and 6th in NA slightly behind MGS4. Even if I'm off by 1m WW it would still land in 5th place.
EDIT: this is probably down from some of my previous predictions. I wouldn't be shocked if this does 10m, but who knows.
