Update: It looks as though we pulled the trigger a little early on thinking authorized devices were limited in the "smartphone" category. After adding 6+ phones to our account, we were still able to stream music on all of them. The post has been updated to reflect this.
Original story: Google Play Music is picking up an update starting today to version 5.6, which includes a few subtle interface changes and some clarity on how authorized GP Music devices are handled. To be clear, this isn't the Material Design-inspired new version of Play Music, but there's still plenty of time for that to arrive and we'll never scoff at a round of subtle improvements to a popular app.
The big change here is to how Google Play Music is now handling "authorized" devices that can connect and play music from a single account. Under the "my devices" area of the settings you'll see the same layout of phones, tablets and computers as before, with the "X" by each one to deauthorize it, but now you'll see phones broken out into their own section from other devices (though some phones we've attached don't seem to filter into the phone category correctly). Google is still allowing 10 total devices be authorized to your account, regardless.
On the design side, swiping in the left-edge panel offers the now-standard Google Account switcher at the top, and a new toggle to switch over to on-device (they call it "downloaded") music only further down — a setting that was previously kept in a selector in the top action bar. (You'll also note that the "Explore" tab grays out when you switch to on-device music, as you can't stream anything.) There's also a new bit of nicer artwork in the download queue section of the settings when you're all done downloading — a little glimpse at some of the simpler Material Design artwork, really.
Be sure to pick up the latest update to Google Play Music from the Play Store link above.