The major changes for 1.3.6.5 Public Beta are:

• Switched matchmaking from GameSpy qr2 (now RIP) to RakNet. BZ2 now uses the RakNet master servers for LAN & online matchmaking, RakNet NatNeg technology for easier hosting w/o having to open ports in many routers, and RakNet communications and connection state management. BZ2 still uses GameSpy for pregame chat (irc) and ingame voice communication.
• Tweaked network code for better performance with lots of players in game.
• Tweaked network code for reduced lag/warp ingame.
• Various other fixes and optimizations.