No way in hell MGS4 outsells SSBB. It would need an attach rate of like 75% to achieve that. But I do think it will quickly become the best seller on the system, even by November, possbily passing GTAIV.
Lifetime I think GTAIV will sell more on the PS3 than MGS4, but this year, because of Japan, I think we will see MGS4 be the PS3's lifetime bestseller.