Sony will be releasing a single player demo for Killzone 3 on February 9th, but only available to PlayStation Plus member and everyone else on January 16th. The demo will take place in the Icy Incursion level and come in two flavors: 2D and 3D and have a separate download to both. It will also support PlayStation Move.
The open Multiplayer demo will come out next Wednesday in the US and Thursday for Europe. Killzone 3 comes out proper on February 25th.