I also use it, and confirm it uses Qt and QtWebkit. Other than some finicky setup to get it to play local MP3s on Arch, it works wonderfully.

Also, for Gnome-using friends, I created a python wrapper for Spotify to map Gnome's media key events to the type that Spotify is expecting, so that I can use my keyboard to control the music. https://github.com/jreese/spotify-gnome