Soon, users who opt for the free (ad-supported) version of Spotify will finally be able to stream music via their mobile devices, according to a report from The Wall Street Journal.

Spotify will reportedly announce the new free tier feature tier at a December 11th press event, an event for which they distributed invites earlier this week.

According to the WSJ, Spotify has been trying to work this whole thing out for almost a year – negotiating with major record labels to determine exactly how much they’ll have to pay to stream songs, free to users, via mobile.

Free Spotify users shouldn’t get too excited about the new mobile offerings – it’s not going to be unlimited. The WSJ says that the new feature “will allow nonpaying mobile users to play a limited number of songs on demand, but will mostly serve up music based on the user’s input” – kind of like Pandora or other radio competitors. Tech Crunch says that free mobile listening will be less-restricted if users choose to listen to previously-curated playlists.

Of course, this marks a significant departure from Spotify’s long-standing model which forced users to opt for the $9.99 per month tier if they wanted to stream music via mobile. Spotify’s other $5 per month option allows user to ditch the ads, but still doesn’t offer mobile streaming.

Spotify has around 24 million total subscribers, about one-quarter of which pay for the service. The move to open up mobile streaming to the free tier is an obvious answer to the challenges of rival services like Pandora, Rdio, and iTunes radio. But by placing restrictions on free mobile listening, Spotify is ensuring that people will still have a reason to opt for the $10 per month plan – so that they can stream any song, at any time, as many times as the want.

Image via Björn Olsson, Flickr