I'll only get this game if Sony sends me a copy for review. No co-op? Interesting. Bioshock has no co-op and I haven't touched it since I finished it. No need - but Halo 3, Gears, Gears 2, COD4 shine online. Unless the game totally rocks, I wouldn't really buy a FPS that doesn't have co-op.
Name another FPS that's selling like hotcakes and doesn't have co-op.
It will hurt sales, IMHO.