Dial M for Music

Mobile-phone makers scored a surprising hit four years ago when they introduced handsets equipped with tiny digital cameras. Today, nearly one-third of the cell phones sold worldwide do double duty as cameras, and that could reach two-thirds by 2009, according to the Boston-based research firm Strategy Analytics. So what will the mobile-phone industry do for an encore?

If you listen carefully, you can hear the answer reverberating from those distinctive white "ear-bud" headphones attached to Apple Computer's iPod digital-music player. After the booming success of the iPod and Apple's iTunes Internet music store, the mobile-phone industry is keen...