Shield: Initially wood (only because leather isn't a selectable shield material)
Not to nitpick, but I guess you didn't know: Leather can be used for shields. In the Leatherworks, near the bottom if you scroll all the way down or reverse-scroll up, it's there. You can also queue up jobs for leather shields in through the Manager interface. Thanks for the replies about layering.
EDIT: To get this thread back on topic, I had an idea:
This... is... GIRDERGROOVES!
1) After your fort's basics are setup (farms and minimal workshops), throw every dwarf into military training (6 of 7 so someone can keep food and booze going) until the first migrant wave shows up.
2) Assess all of your dwarfs at this point, migrant wave and original 7 both. Anyone with military prowess gets the nickname 'Spartan', everyone else gets the nickname 'Slave'.
3) Spartans train non-stop, live in a separate burrow, have personal grand bedrooms, and their own legendary dining hall sustained by the Slaves, which is the only contact the Spartans and Slaves have.
4) The slaves have minimal everything, living in a giant dorm and toiling all day for the Spartans.
5) Slaves have their own militia, clad in leather and armed with bone, wood, copper or bronze. Only Spartans touch iron or steel.
6) As Slaves train, if one begins to show promise and stand out from its peers, they may ascend to Spartan-hood.
If I do this, and I think I definitely will, I'll try to write it up somewhere around here.