As I don't know if Spotify will announce this officially in the proper forum, I thought it should at least be mentioned here. There is a news article on the Spotify U.S. blog though. See also this announcement in the Feedback & Ideas forum on Last.fm. I also copied the Spotify part without the new YouTube player announcement to the Last.fm for Spotify support group. Last but not least you can also read about it on the Last.fm Developer blog.
In short: it enables on-demand playback on all Last.fm web pages showing tracks, i.e. your loved tracks, playlists, personal tag radios, personal or artist track charts, albums etc. You can use it for free with the Spotify desktop software or their web player and do not have to subscribe to Last.fm for using this feature.