Killzone 2 demo will get an opportunity to try it out tonight, when the Helghast invasion of Home begins. If you're not into pre-ordering or setting up a European PSN account, you'll want to be wandering around Home from 9pm EST onwards. At that time, the Helghast soldiers will appear, brandishing demo codes for all who request them. Use the code to redeem the demo from the PlayStation Store.
This may be the last chance to get hold of the demo before it finally releases to everyone on February 26th, a day before the game's actual release. It's not clear how long the Home invasion will last, so procrastinators beware.