I was wondering that myself. Apparently it's just decorative and it actually molded in...



From the Cast Iron Collector: http://www.castironcollector.com/hammered.php
"If you have collected vintage cast iron cookware for even a short time, you're bound to have come across a pan with a "hammered" finish. Like plating, a hammered finish was intended to give a more upscale appearance to the cookware. The dimpled finish mimicked that of more finely-made service ware.
Hand hammering was a common decorative technique on pieces made of softer materials like brass, copper, and tin. The metalsmith would spend a great deal of time and effort to meticulously create the hundreds if not thousands of same-sized impressions on each piece.
"Hammered" cast iron, and cast aluminum as well, differs in that the dimpling is molded in, rather than applied afterwards. The type of iron typically used to make pots and pans is not malleable enough to be hammered, to say nothing of the time and enormous expense a skilled craftsman's labors would add to each relatively inexpensive piece. Therefore, it is the pattern which is hammered, and whose characteristics are then transferred to each mold.
Among hammered pieces, skillets and dutch ovens were relatively common, often being plated as well. Other hammered pieces more rarely found include waffle irons and corn stick pans.
Variations in the hammering style are seen from one manufacturer to another. The dimple edges may be sharp and distinct, or more subdued. Chicago Hardware Foundry and Wagner pieces typically have larger indentations, about 1/4" to 3/8" in diameter."