I had assumed that you have no HW firewall enabled, but reading the post describing your layout again, you probably have a firewall in the LinkSys WRT54G and another separate hardware firewall you are not using.

>>>1 - LinkSys WRT54G Router (WPA configured and connection filtered by mac address)
>>>I do have a separate hardware firewall, but I choose not to use it.

>The firewalls are on separate pcs.

>>Why 2 different firewalls ? AFAIK heightens the risk of intrusion while perhaps identifying more attempts to "phone home" when having identical SW installed.