You connect the client library to a main loop, and register as a listener via the broadcast_ playback_current_id method. If you want the currently playing id when the script starts as well you can call the playback_current_id method.
Here is a small adaptation of tutorial6 in the xmms2-tutorial.git which uses the GLib Mainloop to drive the connection:
import xmmsclient
import xmmsclient.glib
import os
import sys
import gobject
def cb(result):
if not result.is_error():
print "Current: %(artist)s - %(title)s" % result.value()
ml = gobject.MainLoop(None, False)
xc = xmmsclient.XMMS("stackoverflow")
xc.connect()
conn = xmmsclient.glib.GLibConnector(xc)
xc.broadcast_playback_current_id(lambda r: xc.medialib_get_info(r.value(), cb))
ml.run()