Concerning the official arm libspotify the terms of use states that it can be used on an approved device, and (quoting) "Approved Device does NOT include any mobile wireless handset or any other internet-enabled device that is designed to be handheld". I would assume N900 belongs to that category.

Edit: Also noted that it support bitrates 160k and 320k (not low bitrate 96k currently supported in QSpot via libopenspotify).