Sony says Gran Turismo HD will be made available as a free download at the PlayStation Store on December 24.
The real driving simulator will be available to download for a limited period, and will feature a choice of ten cars and two layouts on a single course. There will be two modes of play included--Time Attack and Drift Trial mode. The demo will also have a Network Ranking feature, presumably for the Time Attack mode.
Running up to the release of Gran Turismo 5, the "online car life simulator", it's possible a number of other concept releases will be released, but nothing is set in stone right now, including the release date of the finished game.