Having created a new sd card with the 0.4b image I found that Playlists wouldn't save. I had installed the addons for Lyrics, Youtube and to upgrade the MPD, which may be relevant.
Looking at the /var/lib/mpd/playlists directory using WinSCP I spotted that the 'Owner' was shown as '45', which seemed odd. I changed this to mpd(1000) and voila, Playlists now save.
This may help someone in the future.