Heh, I heard it is due to the UK TV deal. Not sure which network it was, ITV I think.
Before it is available to buy it must be shown and repeated by the networks that licensed Dexter, so that they get their moneys worth.
A baffling way of doing business considering that it will be out 14 months after the series ended on FX in the UK. Most folks would have aquired, imported, torrented, car boot saled series 3 by then :(