Rockstar have updated the official site with details of updated multiplayer modes (which you can read about below) as well as 10 brand new screenshots.

Deathmatch and Team Deathmatch have the following features added:

• health and armor bonuses for ending kill streaks, as well as money
  
• Make an extra for every assisted kill. You can also earn money for ending other players' kill streaks.
• New locations, vehicles and weapons (including the parachute) are also available.
  

Race and GTA Race information and screenshots will be coming soon, as will a video showing it all off.