It depends on the game really, with sequels like Halo 2 and 3 and Gears of War 2 the highest difficulty damn well better be available from the beginning. I've already played and beaten their predecessors on the highest difficulty by this point, so I'm fairly well off for the next game.

But if it's a stand alone game like Bioshock then I don't really have a problem playing through again, especially since Bioshock was very good.