It appears the latest issue of PlayStation: The Official Magazine hints that Killzone 3 is "deep in development" for PS3 and readying itself for 2010, alongside a follow up to Call Of Duty: World At War from Treyarch and Resistance 3.
If Killzone 3 were to launch in late 2010, it would be an approximate 18 month turn-around for Guerilla, certainly not out of the realms of possibility with the engine already complete.
To quote: While reading a copy of Playstation: The Official Magazine (December 2009) issue that was sent to my mail, something really seemed to catch my eye. On page 20 of the magazine there is an article titled "The Secret Games of 2010? and it just so happens to speak of Killzone 3.
"Insider slips reveal three big names you can already pencil for the next year. Killzone 3 is reportedly deep in development, Insomniac's Full Moon Show podcast hints at a project we can only conclude is Resistance 3, and a Treyarch employee's online mentions a Call of Duty: World at World sequel."