FryWalker from XDA-developers forum has made a Spotify client for Windows Mobile. The program is still in beta, so it’s not bug-proof.

Features:
> Full implementation of the Spotify search and top list functions
> Play list support – easy access to all your favourite tracks (up to 150 tracks per play list)
> Choose your own media player – define how each track is played (GSPlayer (free) and PocketPlayer are recommended*)
> Album artwork shown by preference

As with every other Spotify program, a premium subscription to Spotify is required for this to work. Download the program here.