That is disappointing. Have to say though that, on reflection, I'm not surprised. When the original Panda brochure came out they were banging on about the sliding rear seat, but it wasn't available for the first batch of cars built (like ours).
Originally this was supposed to become available last autumn (I remember reading somewhere that it would be an available option in September or October 2012?). I can see on the configurator that it is available to order now for the regular Panda Easy or Lounge.
Not sure what the issue is though, can only hope for your sake that it is something which will be resolved in the immediate future.