I figured out a way to fix the airport issues after upgrade to Mac OS X Snow Leopard. Just insert a bootable Mac OS X install dvd (not the one from Snow Leopard) and boot with C key pressed. Run the Network Diagnostic and connect as usual to your wireless lan. If this succeed, restart your system. That’s it. Your system should now connect direct to your network again without any issues.

After a router reset I had to use this way again. Don’t know why, but it works well for me.