Gran Turismo 5 was supposed to be out in March 2010, but it was delayed. The new release date changed to November. Then the game was delayed again to this "holiday season". The game's delay was blamed on "production difficulties".
Sony hasn't officially revealed the release date yet, but Japanese online retailer DMM.com went ahead and dated the title for December 23. Retailers giving games release dates is nothing new as those release dates are typically place holders. However, DMM.com later yanked the release date from its sites as evident in the link below. The link might've been pulled for any number of reasons.
While December 23 is this Christmas season, this DMM.com listing certainly is not official confirmation by all means. The Christmas shopping season starts earlier in the West, so perhaps the game will hit before then. In Japan, however, Christmas is an important gift-giving holiday, but New Year's is far bigger.
Kotaku reached out to Sony for comment and will update this post should the company reply.