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--OkeyDokey-- said:
They probably delayed killzone so it doesn't clash with Resistence 2.

That's basically it. 

From videogamer.com:

The booth was manned not by staff from developer Guerrilla Games, but by SCEE staff, staff who were perfectly knowledgeable about the game but perhaps a little ill-equipped to answer questions on the delay. "It's to do with our publishing schedule," said one. "We saw our line-up for 2008 and thought we could push it back."

I seem to recall GameSpot mentioning that it was delayed because of "stiff competition" from games like Resistance 2 and Gears 2, specifically.