Initially, American PS3 owners wanting the Killzone 2 demo had to preorder the game to get access. Didn't go down very well. So now, you can get hold of the demo for free.
Consider this a backtrack. Of sorts. Sony announced today that if you're set up with a Home account, and pop into the service for a visit, you'll be able to pick up one of a limited number of download codes for the demo. All you need to do is find someone in Home dressed as a Helghast soldier and ask them for one.
These Helghast missionaries will be in Home from 6pm Pacific onwards. You should do your best to get one, if only to reduce by one the number of people attacking/defending the game with their very lives despite never having played a second's worth of it.
If you miss out, or conversely can't be bothered logging into Home, remember: users with a PAL account (ie a British, European or Australian PSN account) can download the demo no strings attached.
Warning: Helghast Invasion of Home imminent [PlayStation]