Some users have reported loss of internet connectivity after the update to Mac OS X 10.6.4. These users have active IP addresses but cannot access any outside network services.
Fixes
Set manual IP address. It appears that some instances of this issue are caused by DHCP problems and can be resolved by manually setting the Mac’s IP address; follow these intructions.
Boot in Safe mode and apply combo update. Booting into Safe Mode may temporarily restore your Internet connection, at which time you can reapply the Mac OS X 10.6.4 combo updater, which may resolve the problem. To boot in Safe Mode, hold the Shift key while your Mac is starting up. After booting in Safe Mode, download the Mac OS X 10.6.4 combo updater, which weighs in at a hefty 887MB. Quit all open applications and run the installer.