Mercora has joined the burgeoning mobile music fray in what looks to be a fairly big way today, launching its minimally-named "M" service for Windows Mobile 5.0-based devices. Unlike other mobile music services, which'll sell you songs or streaming radio, M gives you over-the-air access to your existing music library stored on your PC, in addition to access to a slew of radio stations. And, in a move sure to please the geekier among us, Mercora has chosen to use an optimized version of Ogg/Vorbis to deliver high-quality audio while keeping bandwidth usage in check. All this comes at a cost, of course, with the introductory price starting at $4.99/month, and subscription packages coming in at $29.99 for six months, $49.99 for one year, and $99.99 for two-years (you'll also probably want an unlimited data plan with your carrier unless you want a big surprise on your next bill). While not part of the initial launch, Mercora says it'll soon roll out a social network component to the service, which'll let you browse and listen to the music collections of up to five friends or family members. If you're curious, you can get in on a free preview right now (minus the music sharing), just don't get carried away with the data usage if you aren't prepared to foot the bill.