Samsung has confirmed to TechRadar that the Galaxy S III will not be announced at Mobile World Congress. This lines up with what we've heard, and refutes earlier rumors that Samsung would be announcing the device in Barcelona next month. A rumor that Samsung never commented on until now.
What is confirmed is that the Galaxy S III will have its own event, similar to the Galaxy Nexus announcement. Of course, since this didn't come from "sources close to the matter" or "some guy I know who heard from somebody," we should obviously treat this with a healthy dose of skepticism.