I don't remember what all he's filming, I just know he's doing a lot of background/history stuff and side stories. I said maybe Silmarillion or some of the Tales but I don't recall. You could always google it if you want to know for sure.
I was just wondering what backstuff he'd be using from TS because I don't really see them as being related in any way. I think I'm still pressed that they decided to go the 3-film route.
I don't think they'd be using The Silmarillion, but rather The Unfinished Tales (iirc, it talks about Gandalf's meeting with Thorin and how they decided that they were going to try to take back The Lonely Mountain).