I too am concerned, but from what i've seen GT dosent necessarily sell loads in the first 2 weeks. what i mean is that COD, Halo etc will sell more in the first few weeks. but GT has titanic legs. i think part of that 15m who bought GT3 moved to Forza, so yes we could be in for a dissappointing release. not even modern warfare 2 has sold 10m on PS3.
even if it dosent sell 10m, sony will have other chances to reach that 10m with other franchises, at the moment its still a battle to get more consoles into peoples living rooms, i think about 8 mil is right. and if the game sells a lot of consoles, then it could well shoot past 10m.