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Eclipse by default does not ship with the Java Servlet library files.file servlet.jar. As far as I understand it, this is on purpose as this library file resides with your particular application server.
So you must manually do this.
Two techniques here... not sure if happy with it - http://tech.amikelive.com/node-269/eclipse-quick-tip-resolving-error-the-import-javaxservlet-cannot-be-resolved/