On reboot or after long periods of inactivity, my client's OS X 10.4.10 Mac would lose its network connection to his home's wireless network, and he'd have to re-select the network name from the AirPort icon in the menu bar. He had set a 40-bit password, and the non-Apple router was configured with the same 40-bit password.
By changing the router password to a 128-bit password, and changing the password on the Mac to the same, it's now able to restart or go to sleep and re-establish its connection to the wireless router without any problems.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080110061852354