The service to Pembroke Dock started back in the summer. Although it is currently only 1 coach in each direction per day to connect with teh overnight ferry it is popular especially with the cheap through fares.
Megabus have a fleet of double deck coaches which will start arriving at stagecoach wales Cwmbran depot from this december to boost capacity on the services which have become very popular (That said I wonder how 1 of these double deck coaches will manage to get round Carmarthen)