When Joe Ryan started talking about his plans to open a brick-and-mortar store filled almost exclusively with iPod accessories, people said he was out of his mind. After four successful years heading up an Apple retail site, Think Different Store, the idea of launching a walk-in store sounded backward, if not risky. Now, however, with one busy 3,000-square-foot iPod accessory store and another one in the works, Ryan just might be leading a new trend.