Some computers, after upgrading to Mac OS X Tiger 10.4, may no longer automatically join a wireless network after waking from sleep or restarting.
This may happen when a computer has been upgraded from an earlier version of Mac OS X, or it may happen if your new computer (running Tiger) has had data imported from an older computer.
Update your computer to the latest version of Mac OS X Tiger 10.4 using Software Update or by manually downloading the update from http://www.apple.com/support/downloads. After updating Mac OS X, the computer should automatically rejoin the selected network after restarting or waking the computer up.