With FFXIII already delayed until next fiscal year (AFTER March 2008), an MGS 4 delay would leave Sony without it's two biggest sure-fire blockbusters that were supposed to come out '07.
I don't know actually... maybe Sony's lineup of War Hawk, Little Big Planet, Home, Heavenly Sword, Lair, Uncharted: Drake's Fortune and Rachet & Clank 3 will be the sort of lineup that actually doesn't need help from MGS 4 at all!