Bungie.net : Well, that just happened
Concept art and some plot info is on IGN: Bungie's Destiny Story Details, Concept Art Leaked - IGN
According to the document, “Our story begins seven hundred years from now in the Last City on Earth, in a Solar System littered with the ruins of man’s Golden Age. A massive, mysterious alien ship hangs overhead like a second Moon. No one knows where it came from or what it’s here for, but only that it’s our protector. Meanwhile, strange, alien monsters creep in from the edge of the universe, determined to take Earth and the Last City. We are young ‘knights’ tasked with defending the remains of humanity, discovering the source of these monsters and – eventually – overcoming it.” Elsewhere in the document, the massive alien ship is referred to as “the Traveler.”
Oh, and Bungie was apparently dropping hints as early as 2009.