Some users have reported an issue in which AirPort/WiFi connections repeatedly drop after the update to Mac OS X 10.6.5, or cannot connect to previously functional AirPort base stations and other wireless routers. In most cases, users can temporarily resolve the issue by turning AirPort off then back on.
Fixes
Change 802.11 spec on router. Try changing your router’s wireless spec mode from B, G and N to G only or vice versa. 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.
Update router firmware. Check your router manufacturer’s website, and download and install any new updates for your wireless router.
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.