Lucasfilm have officially clarified their position on what is and is not Star Wars canon moving forward, and it's sort of what it was before, but with a twist.
Lucas always made it clear that he was not beholden to the EU. He set the films he created as the canon. This includes the six Star Wars episodes, and the many hours of content he developed and produced in Star Wars: The Clone Wars. These stories are the immovable objects of Star Wars history, the characters and events to which all other tales must align.
Well, "immovable" until George himself decides to tinker around with them - Greedo firing first sure wasn't immovable. I suppose that goes without saying. This is the first absolute confirmation of Clone Wars as canon, which was not necessarily a given, as the Ewoks and Droids cartoons don't really fit any more. Nothing is said specifically about Shadows of the Empire or The Force Unleashed, which seemed to be kinda-sorta-maybe canon when they came out, or the Darth Plagueis novel that a lot of you treat as canon based on your comments.
The biggest news is that moving forward, ALL new E.U. will be coordinated and canonical, starting, presumably, with Star Wars Rebels (and, one would imagine, excluding Star Wars Detours when that eventually sees the light of day). And it will mine the old stuff for ideas - The Inquisitor, so they say, comes from '80s RPG material.
This is good and bad - good because coherence is nice, bad because when they get M. Night Shyamalan to do a Yoda spin-off origin movie or whatever, every part of that becomes official backstory.
Here's a video where good Lucasfilm employees explain why their employer has chosen correctly.
h/t John Kelly