The only way you'll be able to play Killzone 2 before the game's release will be through GameStop's exclusive pre-order campaign. No other demo for Killzone 2 will be released before the game's street date. Those that don't want to pre-order from GameStop will have to wait until after the game's release, as we've discovered from SCEA. "The demo will be available on the PSN after the game ships but only through GameStop until then."
Seems a bit strange to reward people already ready to purchase the game with a demo, while excluding those that are on the fence. Having played the game, we can confidentally say that playing even just one level of Killzone 2 will make you a convert.