So for $15 per month (the portability-enhanced subscription costs a premium), consumers will be able to fill their MP3 players with thousands of songs. Doing so on an iPod through Apple's iTunes would cost tens if not hundreds of thousands of dollars.

It's pretty compelling.

But there's a problem.

Consumers who have signed up for subscription services generally love them and sing their praises. (Full disclosure: I'm one.) However, the services have yet to craft a marketing message that breaks past the early adopters and into the mainstream.

Apple's iTunes, for example, announced it has sold 250,000 million songs since the store went online, and the iPod -- which only works with the iTunes software interface -- holds more than 90 percent of the portable music player market, according to NPD Group. All told, the leading providers have less than two million subscribers among them.

In part, this popularity gap is because signing up for a subscription service requires a completely different relationship with the content. Subscription services allow consumers to rent music, as opposed to own it. Introducing portability on top of an already foreign idea will require a significant expenditure of marketing and advertising dollars -- something some services aren't willing to do.

"We're not going to advertise this feature," said one representative of a leading service. "It's something that will take time for consumers to grasp."

And the software partners providing support for these services isn't likely to help in the promotion either. Microsoft, which provides the Janus technology that makes the portability possible, is more focused on promoting its Plays for Sure initiative, which leaves online retailers on their own.

Ellie Hirschhorn, executive vice president and chief operating officer at MusicNet, the backend provider for several subscription services, agrees that crafting a simple and powerful marketing message will make a big difference in consumer adoption of portability.

"Marketing is the key," Hirschhorn says. "The technology aspect needs to be stripped out of the equation. People need to understand that they can take their music with them, they can take it on the bus."