Awesome song, that. It seems to be in the key of Db (minor, blues, ie, the Db minor pent scale). Tuned down, as he is, it's in D.
The basic riff goes something like this (tune down a semitone):

---------------------x----
---------------------x----
---------------------x----
---------------------x--3-
5-5p3-5p3-------x---
---------------5-3--x----
Hope this helps.