MGS4 sold to 12% of the PS3 userbase first week. If Killzone 2 did the same (assuming a 23M userbase) it would sell 2.76M first week.
I don't think it will get the same percentage as MGS4 though because:
As the userbase expands, attach rates decrease.
It doesn't have a particularly dedicated fan base, and certainly not as dedicated as MGS.
People are more uncertain about the quality of this title, so will be more hesitant to buy straight away.
What it will have though, is a lot of hype and a huge marketing campaign (I assume), the best looking console game and it looks to be a quality title. To reach 1M first week (assuming a 23M userbase) it would need to sell to just over 4% of the userbase. I think that is garenteed even with average reviews.
To reach my estimate of 1.5M sales, it would need to sell to just under 7% of the userbase. This is certainly possible with great reviews and good marketing.
