Rockstar has updated the official website with the first multiplayer details. The page confirms that Deathmatch, Team Deathmatch, Race, GTA Race and Free Mode have all been updated to include new features.
Deathmatch and Team Deathmatch contains two new gameplay features - kill streaks (receive health and armour bonuses for kill streaks of 3, 5 and 10) and assisted kills (make $25 for every assisted kill). You can also earn money for ending other players' kill streaks. New locations, new vehicles and new weapons (including the parachute) are also available, Rockstar has confirmed.
Coming soon: Race and GTA Race information & screenshots, plus official multiplayer video.