The first instalment of Xbox 360's exclusive episodic content has been delayed from August 2008 to some time after November, according to a Take 2 press release.
"Fiscal 2008 guidance reflects the release of the first installment of episodic content for Grand Theft Auto IV for Xbox 360 in the first quarter of fiscal 2009 instead of the fourth quarter of fiscal 2008 in order to provide a better balance in Take-Two's release schedule."
Not much has been said about the content since the original announcement was made last year, but the episodes (of which there are several) are believed to include things like new weapons, new races and new multiplayer game modes. More info will hopefully be revealed soon.
Link: Take 2 Press Release
Related Link: GTAForums Topic