The release of the new mobile clients follows the addition of web streaming for Xbox Music pass holders earlier in the year.
The web streaming option has also been updated, you no longer need to have an Xbox Music pass to stream music for free, but you will have some in-stream advertising to listen to (much like the free web based version of Spotify, and others). Microsoft have said that the streaming will not feature a 'time cap' limiting your use of the service for at least six months.
Of course Microsoft is going to have two music services under the Redmond Roof after the purchase of Nokia's Device and Services division is completed. Nokia Music is more akin to a streaming radio service, as opposed to an 'all you can eat on demand' service that the Xbox Music Pass offers, so it will be interesting to see if they remain separate, or combined at different tier levels of pricing (Xbox Music is a monthly subscription service after a thirty day trial, Nokia Music is a free service but has an enhanced premium level for a smaller monthly fee).
Arguably Microsoft should have opened up the music subscription service two or three years ago. If that had been the case, the Zune Pass (as it was then known) would have been ahead of services like Pandora, Rdio, and Spotify, with wider coverage and the revenue of Microsoft behind it.
Still, it's here now, so set your music free (into a slightly bigger walled garden of devices) for the same monthly cost.