Will be available sometime in December from what I've read.
Edit 2:
I just copied this from the us.gran-turismo.com site...
Remote races can be started from the us.gran-turismo.com official website, where you can even monitor the progress and check results of the race.
While on the go or away from home, you can check race results, and even set and start new remote races. A whole new way to enjoy Gran Turismo is here.
*The service for remote race is planned to made available in December 2010.
* To host a remote race, you must be signed in to the PlayStation®Network.
*This feature enables the remote operation of "Gran Turismo 5" from a web browser, and requires "Gran Turismo 5" to be running on your "PlayStation 3," set to the remote race screen in the game.
*Race progress is displayed via text only.