We are seeing more and more services move from a dedicated desktop app to more lightweight browser based apps lately. Spotify has just moved its browser based app into a "large beta testing phase" in the UK, and we could see the full browser app take over by the end of the year.
The browser based app has been in a small closed beta for some time now and has only recently opened up to a much larger user base. A representative for the company said "We're letting a number of users in the UK test out a beta version of our basic web player, which we're gearing up to release later this year".
Our UK readers can test out the new interface by heading over to play.spotify.com and signing in with their account information. The interface is familiar and is almost identical to the dedicated desktop app we are all use to. The only difference I can see is that the actual player controls have been moved to the right-hand side of the screen.