This is a nice win for Spotify, and a huge improvement over their previous embed pages. As a Spotify Premium member myself, I’ll appreciate being able to play tracks and playlists on several music blogs I follow. However, in practice the Play button is more a triumph of marketing over real technological progress; with the full desktop still being the player’s audio source is a disappointment. Furthermore, it could easily frustrate those use to clicking on self contained flash music widgets.
A great solution would grant limited access, like playing one song via embed, followed by a prompt to log in and hook up via the standard app. That’s a potentially tricky technical implementation, but a simple IP check could just register number of plays before presenting the paywall.