ps2.ign.com has a rumor that Killzone will be delayed.

While Killzone's delay could likely be blamed on the huge lineup of PS2 software already planned for the winter months and the probability that it may have to go head to head with Halo 2 on the store shelves.
I don't mind if they delay it because it isn't finished. I'd rather the game not be rushed to market if they can make it better. However, according to the article, Sony may hold it back because of the big titles coming out this fall, such as Halo 2. Now, call me stupid, but wouldn't you want your Halo killer app coming out at the same time as Halo 2?

This is only a rumor however...

Here is the link:
http://ps2.ign.com/articles/536/536843p1.html