RealNetworks will announce a revision to its Rhapsody music subscription service that lets users transfer their tunes to portable devices.

The New York Post warns that the new service - which emulates Napster To Go - could "threaten the popular iTunes service". Tracks will be playable on devices from Creative, iRiver and Samsung. The service uses Microsoft technologies.

Real's new Rhapsody will offer subscriptions for $15 per month. The company will reveal its news at a press conference today.

"Portability is the single biggest threat to Steve Jobs' iTunes," one high-level record executive reportedly said.