The patch brings better-looking shadows as well as performance optimisations and bug fixes, and finally removes the need to run that irksome Rockstar Games Social Club application before you can play the game. The full changelog follows below.

GTA IV Title Update 1.0.6.0 \ 1.0.5.1 \ 1.0.4.2 (Patch 6)

• Rockstar Games Social Club
  • Remove Rockstar Games Social Club application
  • Separate RGSC application no longer required to launch game
  • Social Club login now occurs during the launch process
• Shadows
  • A new, less memory-intensive, and better-looking scalable shadow solution has been implemented
  • Enhanced Night Shadows added with user-selectable level of detail (replaces Shadow Density)
• Performance
  • Rendering optimizations have been made to improve performance, particularly when enabling shadows
  • User-configurable graphics settings have been added for shadow control
  • Improved memory management
• Multiplayer
  • Enhanced security to isolate users from cheaters and hackers
• Episodic Content
  • Provide in-game support to purchase "The Lost and Damned" and "The Ballad of Gay Tony" (Not available in Russian/Japanese Versions)
• Bug Fixes
  • Vehicle shadows re-enabled between 8pm and 6am
  • Fix floating street textures
  • Pedestrians in vehicles appear further out from the player to allow for long range sniping
  • The Moon has been implemented with simulated lunar phases
  • Fixed garbled text in Japanese Multiplayer menus
  • "RESC 10" error fix for specific hardware configurations

Thanks to 'Pulverdings' for the tip.