Yeah you need to have Spotify Premium account to be able to use it in a cellular.

A thought came to me: what if you use "Hide user agent" and connect over wifi? Would the Spotify servers know what device you were connecting with? I have a premium account now so I can't try it out.

Another thought related to improving Yaspot: Whenever you open a saved playlist, jump into the "Playlist" view where you have the Pause\Play etc. buttons. Also, a shuffle feature would be nice.