Eli Dourado examines why Apple would bother with free engraving and digs up a thought provoking argument for customization. As a larger pattern it is good to remember that that a broader set of tactics are required to protect a lead in the market.
The real reason Apple offers free engraving is to weaken the secondary market. iP*ds are durable goods. Apple has a monopoly on iP*ds, but it still has to compete with the products of its former self. If people get tired of their iP*ds or decide they want to upgrade to a newer model, they can sell their devices to other consumers, who in turn are not giving their money to Apple. By offering free engraving, Apple makes these used devices less valuable to other consumers. Who wants a weird engraving chosen by the previous owner on his iP*d? The more iP*ds are engraved, the smaller (or at least less valuable) the secondary market is, and the more profitable it is to be the durable-goods monopolist, Apple.