It's a story that he's been working on forever that he hasn't, until recently, felt ready to actually write. He's always known that this is HIS centerpiece story. He didn't want to seriously sit down and try to make it come to life before he was ready as a writer.

I believe he envisions it as a monster series too, something like 10 books. Involving a large number of different story lines, magic systems, etc. It's been a while, but I know he had some stuff about it on his website before Warbreaker came out.