Add some information, and a log message letting you know it worked

fml/common/cpw/mods/fml/common/registry/GameData.java

@@ -354,6 +354,7 @@ public class GameData {
             try
             {
                 Files.write(mapJoiner.join(builder.build().entries()), f, Charsets.UTF_8);
+                FMLLog.log(Level.INFO, "Dumped item registry data to %s", f.getAbsolutePath());
             }
             catch (IOException e)
             {

fml/common/cpw/mods/fml/common/registry/GameRegistry.java

@@ -411,6 +411,9 @@ public class GameRegistry
 	 *
 	 * If it is built from a block, the metadata is by default the "wildcard" value.
 	 *
+	 * Custom itemstacks can be dumped from minecraft by setting the system property fml.dumpRegistry to true
+	 * (-Dfml.dumpRegistry=true on the command line will work)
+	 *
 	 * @param modId The modid of the stack owner
 	 * @param name The name of the stack
 	 * @param stackSize The size of the stack returned