A small update was released to Google Play Music yesterday (the changelog just posted today), adding a brand-new feature to the service's radio function. You can now start radio stations based on playlists, as opposed to artists or songs. This will certainly be a welcome feature if you've already got a library of playlists set up, though if you don't utilize them there's probably not much of a reason to start.
The new version, 5.4.1413N, should be available via the Play Store for most people at this point, but just in case you're not seeing it, you can download the APK directly at the mirror below. Google added genre-based radio stations to Play Music back in September, and "I'm feeling lucky" radio in October, so today's change just seems to be adding greater richness to the feature.
Download: Google Play Music 5.4.1413N
Thanks to everyone who tipped us on this!