There have been many ways for people to gain access to the Killzone 2 demo before the game actually comes out in about 10 days, from now. Most recently one could have hunted down one of the only Helghast avatars in PlayStation Home and request one from them. Apparently something that was difficult for a bunch of people on the PlayStation Blog. But outside of that or pre-ordering the game, like one should, Sony is hooking everyone up again with a chance. This time through Twitter.
If you have a Twitter account, first start following pwnedcom and then follow SonyPlayStation. Once you have that going, send a Reply to SonyPlayStation stating you would like one of the few hundred demo codes for the game. Once they get into their offices today and if they have some left, they will send you a code so you can join in the early fun to get pumped for this game. So get to tweeting to get hooked up.
Here is an example of what you should send them.