Installing portions of games to the hard drive of the PlayStation 3 has become a standard. Different developers use the feature to varying degrees, but almost every new PS3 game has a mandatory HDD install.
What about Killzone 2, you ask.
It won't have one. At all. Not even an optional install.
Seb Downie, QA Manager for Guerrilla games, said, “No install... optional or otherwise.”
We're guessing this means that they have managed to effectively optimize the game to work around the Blu-Ray's slow read speed and do not require assets to be pre-loaded to the hard drive. We do expect, however, that the game will definitely use the hard drive to cache data during play.