Following a security update, my PowerBook kept losing its wireless connection when waking from sleep or starting up. 
This was a pain as my WiFi base station doesn’t transmit its SSID and I had to enter its name and long hex WEP key each time I wanted to get online.
Turns out the solution was rather simpler than all the mucking around I’d been doing deleting com.apple.airport files and the like.
I just made a new ‘location’ – its location had been on ‘automatic’, so I went to System Preferences > Network and clicked on the locations drop-down menu and made a new location (called ‘foo’ of course). Since then it’s been rejoining my wireless network ok when it wakes up. Phew! Or should I say ‘Foo!’?