General consensus seems to be that the core rulebook, Scum & Villainy, and Starships of the Galaxy are the most essential. The Force Unleashed also has some prestige classes and rules additions that make healing easier, make droids more playable, and add crafting rules to the game.
Just a heads up, Starships of the Galaxy and the Knights of the Old Republic Campaign Guide have become difficult to find, so expect some sticker shock if you go looking for those books online.
I don't recall any mention of it over at the Wizards Star Wars forum prior to Rodney's tweet about it. Given that a number of folks over there are as much fans of Rodney himself as of Saga, I'd be surprised at people keeping quiet about it. Also, Rodney's tweet made it sound like it had been a secret before GenCon, since Dark Sun itself wasn't announced until then.