My Powerbook suddenly no longer finds the Airport Express network I've set up, and neither does the Airport Utility think that there's an Airport present. The device is turned on, though, showing the green light, and the DSL modem to which it's plugged in also shows a green light to indicate that a valid device is connected. Restarting the Airport and rebooting the Powerbook made no difference. Is there something else I can check? The only recent system changes where the iTunes, Safari and Tiger Security updates.