Spotify streams in the OGG Vorbis format, which is not supported by Roku. If you want to listen to Spotify on the Roku you need to convert to MP3 on your computer and stream over HTTP to the Roku. I suppose it would be possible to configure a channel that could provide a UI for Spotify, but the main problem is that you need a transcoding server running while you listen.
The most comparable alternative available on Roku is MOG.
If Spotify decides to offer MP3 streaming sometime in the future, then a dedicated Roku channel would be possible.
Roku HD N1000 Serial #20G984014682 Software V. 3.1 Build 1167