According to this tweet from Brendan Arndt, the Device Portfolio Manager for Three UK, Spotify is due to arrive on Windows Phone 8 very soon – even as soon as this weekend. He mentions in a following tweet that it has ‘apparently been submitted for approval’. WMPoweruser reports that approval for new apps has been very quick recently, as little as two hours between submission and the approval of the app, so we could see the new Spotify app in the store in just a few hours as this post goes live.
Xbox Music and Rdio (amongst others) are already established on the platform, but there has definitely been a significant interest (and want) for Spotify, due to its popularity as a social network and familiarity with a large number of users.
Update: Spotify is now available in the Windows Phone store!