Oh, also if AirPort wireless is your primary means of connecting to your LAN, check in System Preferences >> Network >> Network Port Configurations to see that AirPort is dragged to the top of the list. This ensures that AirPort is given priority over Ethernet, Bluetooth, and FireWire.
While you're in there, it never hurts to make a new Location -- disable all nodes except AirPort, drag AirPort to the top of the list, then in AirPort Configuration set your wireless network as preferred.