first off, what is the timeframe? 2 weeks? 1 week?
MGS4 is at 1.8m 1st week, so no way it gets the title of fastest selling.
Uncharted 2 was 780k 1st week, and had good legs due to the holidays so if we're counting multiple weeks it will give GoW3 a run and probably beat it out.
KZ2 came in at 750k 1st week, but the legs were only ok (compared to Uncharted 2).
750-900k WW 1st week, so it has a shot at #2. It'll be lucky if it breaks 30k in Japan (almost double previous numbers), America probably won't get all that much higher than the pre-order numbers so 500-600k (due to demo access as a pre-order bonus. Also it seems like the more "hardcore" the game, the closer it is to the pre-order numbers). PAL isn't exactly crazy about God of War either, so 200-250k would be respectable for the series.
so, I say it is a toss up for #2, but even if it gets it, it will get destroyed by Gran Turismo whenever that manages to launch.