I don't know what tuning in particular, but its a pretty low open tuning for a lot of the songs. It probably changes depending on the song. There's a lot of banjo in there too, so it could be something related to open G...that's where I would start.
Yeah, I fell in love with Sam Beam's music the first time I heard it. Not bad for being recorded on a 8-track in his living room, eh? I haven't heard his most recent one "Our Endless Numbered Days", but I heard that he actually went into a studio to do it.