Believe it or not, Chemicalkinetics says on Chowhound, a plain old metal spatula is your best choice, even if it damages some of the seasoning you've been careful to build up.
Seasoning an unlined pan is an ongoing process: Over the life of that cast iron skillet, you'll be constantly building up and breaking down layers of seasoning, but that won't harm it in the least. Metal spatulas dislodge burned food residues little by little, Chemicalkinetics says, leaving the underlying seasoning intact (even stronger, in fact). When your pan is new and unseasoned, you'll want to use silicone or bamboo utensils (or metal ones with a very light touch). But after two weeks of diligent seasoning, metal is better.
Chowhound breadchick uses a metal spatula with her carbon-steel pan, which seasoning has made essentially nonstick—the spatula just slides between food and pan, no scraping needed. At the end of a carbon-steel pan's first month, DuffyH says, a metal spatula won't even knick the pan's built-up seasoning.
As for what kind of metal utensils work best, Chemicalkinetics suggests avoiding rubber handles and rivets; for maximum longevity, seek out ones formed from a single piece of metal.
Photo by Chowhound user tcamp