It's real simple, 360 is already over the hill. halo 3 is the biggest selling game the 360 has and ever will have. There are no other exclusives on the horizon that will sell millions of copies. Gears 2 will be a hit, but not as big as Gears 1(it had pretty much no competition when it came out). Ninja Gaiden 2 will do good as will Fable 2, but what else is there?
PS3 on the other hand hasn't even had one major hit released yet. GT Prologue is a good start, buts it's not the full game and GT5 will crush it.
MGS4 will be the first exclusive hit with a history of massive sales. It will sell millions of copies and push console sales(especially with the bundle). Then you have 2 Final Fantasy exclusives that are shaping up nicely. Killzone 2 could also be a huge hit if the gameplay and story are on par with the amazing graphics.
PS3 has been outselling the 360 for weeks in all 3 regions with no big exclusive games. When the big titles hit, it'll leave 360 in the dust.
FYI to the topic creator, there's a good chance ME2 will be multiplat, Bioshock2 will be multiplat, and wtf is halo4???