Looks like Killzone 3 is real, the latest issue of Gamepro apparently has the details.

Expect a feeling that the player will be "in a place far away from home, outnumbered”, there’ll be jetpacks, arctic levels, and – oh – 3D graphics, which require a 3D TV and, obviously, 3D glasses.

The mag hints at a grander scale and a new weapon called "the Wasp”.

I’ve a sneaky feeling Killzone 3 was shown to a few journalists a week or two back, and there’ll be an embargo up soonish.  Just a hunch.