Carriers initially prohibited the practice of allowing users to transfer existing music from their personal digital libraries to MP3-capable phones — a process known as "sideloading" — but they are now warming up to the idea.

Motorola's iTunes ROKR phone is the latest and best-publicized example. Cingular is not only supporting the sideloading capabilities, it is encouraging it with a rash of TV, billboard and print ads mimicking Apple's successful iTunes/iPod campaign.

Consumers in Japan, South Korea and Great Britain are already transferring songs directly from their computers to their phones, a practice known as sideloading, or downloading full-length tracks over their mobile networks.

Ringtones may be big in the United States now, but full-track downloads and sideloading are the future, said Thomas Hesse, president of global digital business at Sony-BMG Entertainment.

With the industry now adjusted to the concept of paid-for downloads, high street chains are steeling themselves against new claims on their market share. That may mean consumers "sideloading" from their digital provider to their mobile phone (witness Napster's new partnership with Ericsson, and iTunes' association with Motorola).