Killzone 3 exists and may be with us in 2010 according to a German web site which lists a Netherlands TV Show as their source. Jan-Bart Van Beek, Guerilla Games animation director, was on a TV show and was asked about Killzone 3.
He said it’s scheduled for 2011, although it may arrive in 2010, but there is no pressure from the publishers (Sony) to aim for this date. The lack of pressure provides the developers time to create something “more than sustained fire”. That is probably Babelfish translation failing but I think it means Guerilla are be given time to create something rather special as opposed to ‘more of the same’.