In 2001, Apple introduced the iPod, an MP3 player with the unheard-of storage capacity of 5 gigabytes. Six iPod generations later, the iPod plays songs, movies, games and photo slideshows, and you can store up to 160 GB of any type of file you want. The evolution has been a lesson in consumer electronics marketing and development: Millions of people are so hooked on the iPod, they continue to buy