This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
The ServletConfig represent the configuration of the servlet from the deployment descriptor. It makes no sense at all for any type of set operation, so there aren't any. If you want to set run-time values, that's something you'd do with the ServletContext.