Cingular today announced the launch of its long-awaited 3G downloadable music service, but instead of running its own branded music store, Cingular is keeping with its open-platform approach and opening up music capabilities to several third-party content providers, including Napster, Yahoo Music, eMusic and XM Satellite Radio.

Cingular is setting up the framework of the music service, equipping five data-capable phones with media players and Microsoft digital rights management and supplying over-the-air streaming and PC-to-phone sideloading capabilities, but the carrier is relying on others for the actual music content. Both Napster and Yahoo Music will integrate their subscription-based music services with the Cingular service, allowing customers to upload millions of songs from their PCs to their phones for monthly fees. In addition, Cingular is partnering with eMusic to purchase music in MP3 format, which can be used on other digital music players, Sony Ericsson Walkman phones or burned onto CD. Finally, Cingular is working with XM to deliver direct over-the-air streaming of 25 audio channels.

Aside from announcing its partners, Cingular gave few details of the mechanics of its service. Cingular did not say if customers’ existing music libraries could be transferred onto the five new music phones—the Samsung SYNC, the LG Electronics CU500, the Sony Ericsson W810i and W300i Walkman phones and the Cingular 3125, a Windows Mobile smartphone built by HTC—though it did say the media players support MP3 encoding, a universally supported digital music format.

The far-spanning service, however, appears to be much more comprehensive than that of its competitors. Sprint and Cingular both offer their own music stores, which allow customers to either download directly to the phone or upload from a PC connection. And though Sprint does offer streaming music and audio, its services do not directly integrate with other music providers’ services. By teaming with Napster--which already delivers music via mobile to SunCom through Ericsson’s content hosting platform--and Yahoo, Cingular can tap into customers who already have relationships with those providers, just as its iTunes-powered Motorola phones are aimed at customers that have relationships with Apple’s music service.

What the service doesn’t yet support, though, is over-the-air downloads of music, an application that both Sprint and Cingular have said has built significant traction. The Cingular music engine will allow customers to preview songs over the air, but actual downloads will have to be done through the PC. Part of the problem may be the lack of an adequate downlink connection. While Cingular is building out one of the fastest 3G networks in the U.S., using UMTS and High Speed Downlink Packet Access technology it is behind both Sprint and Cingular in its nationwide rollout. It also has also been slower in procuring UMTS handsets for the unique PCS and Cellular frequencies used in the U.S. Of the five phones supporting the new music service, only the new Samsung device supports HSDPA’s 400 kb/s to 700 kb/s download speeds. The LG CU500 has UMTS, but the two Walkman phones and the HTC smartphone are only EDGE compatible.