I assume you are booting the Macs to Windows if you are planning to render using 3ds Max. Is this correct?
Are you running backburner as an application or as a service? If it is running as a service, it could be an issue with backburner bonding to the MAC Address of the wireless NIC rather than the MAC Address of the wired NIC.
Are you disabling the wireless NICs when the machines are plugged in? In our office, we have only couple of Macs but had some issues when the wireless NIC was left on and the computer was plugged into the network via ethernet.