According to Gran Turismo's official website (translated), GT HD Concept will be pulled from the Japanese PlayStation Store on September 30th, creating speculation that the same will occur in the US and Europe shortly after.
This move is being made in preparation for the release of Polyphony Digital's newest GT title: Gran Turismo 5 Prologue.
A free demo of GT5 Prologue will be available on the Japanese Store on October 20th, followed by the game's full release on Blu-ray and in downloadable form on December 13th.
As for all you GT HD players out there, don't worry, Polyphony Digital has confirmed that they will continue the ranks and leaderboards.
Stay tuned to PSU for more information on your Gran Turismo needs.