A handful of users have reported Wi-Fi connectivity issues after the update to Mac OS X 10.6.4. These problems include inability to connect to wireless routers (such as AirPort base stations), slow transfer speeds, spotty connections, dropped connections and more.
Reset SMC. Some users have reported that resetting the Mac’s SMC (system management controller) at least temporarily resolves this issue.
Reset router settings. Try resetting your wireless router’s settings. For AirPort base stations, this can be accomplished using the AirPort Utility. For third-party routers, this can generally be accomplished by either holding down a button on the back of the unit (see your unit’s manual for details) or logging into your router’s configuration page by opening a browser window and entering the address 192.168.1.1 (you may need to enter the username: admin and password: password1) then using the reset function.
Change or turn off wireless security You may want to try toggling the settings on your wireless router, switching from WPA to WEP or vice versa, or, as a last resort, turning wireless security off altogether. For AirPort routers, this can be accomplished with the AirPort Admin Utility. For other routers, this can usually be accomplished by accessing the router’s configuration page — open a browser and enter the address 192.168.1.1.