Some users have reported AirPort/Wi-Fi connectivity issues after the update to Mac OS X 10.6.3. These include slow throughput, inability to connect, or a need to persistently refresh Web pages before they will properly load.
Possible solution: Boot in Safe Mode
Booting your Mac in Safe Mode then simply restarting normally is one of the most overlooked, most effective procedures for solving a variety of issues after an incremental Mac OS X update. The reason for its efficacy: booting in Safe Mode forces a disk directory check, clears potentially problematic cache files and performs other routines, detailed here.
To boot in Safe Mode, hold the Shift key while your Mac is starting up. After booting in Safe Mode, simply restart normally (without holding the Shift key) and check for persistence of the issue.
Apple Discussions poster Antonio Barros solved his Mac OS X 10.6.3 Wi-Fi issues via this method. He writes:
“With 10.6.2 on my iMac 21.5″, my Internet connection over wifi was working great. After updating, the service became more and more degraded and eventually I could not use it at all. I re-installed 10.6.1, and updated to 10.6.2 but it didn’t help. In despair, I booted in safe mode, and then booted normally. Since then, I access the Internet flawlessly. I updated to 10.6.3 v1.1 and I still access the Internet over my wifi network flawlessly.”