MSN Messenger 7 Beta Build 604 contains quite a number of changes from the previous beta build (425) including slightly modified icons and a grey text message next to each contact icon's name actually explaining their current status (Online, Away, Offline etc).
Enabling the personal message to tell contacts what you are listening to currently (tick the option) will change your personal message to reflect the current mp3 you are listening too. Interestingly, when tested with a music video file in the current playlist the personal message is blanked out while the video file is being played. MP3 files listed in the playlist following the video file will continue to update the personal message.
FlexBeta
FlexBeta