Quoted:
But yeah, I don't see any use in 'seasoning' a smoker. Your food doesn't come into contact with anything except the grates.
Which is only part of the reason you season a smoker.
How:
When I seasoned mine about 3 months ago, I rubbed it down with lard cakes. It was a little messy, but seems to be the best way to ensure a good coating over all the metal. I rubbed the racks and ALL metal surfaces inside the cooking chamber. After applying the lard, I lit my fire. I let it burn for maybe 3 hours. Once it cooled, the insides looked like a well seasoned skillet.
Why:
Protection and sanitary purposes. My smoker was built from a 250 gal LP tank. There was some rust prior to the seasoning. After seasoning, there was no sign of rust anywhere. The metal surfaces had a dull black finish. I have cooked on it 5 or 6 times now, and the grease from the meat I cook continues to build a hard-surface cake on the previously seasoned metal.