Perhaps I missed something, but how does one use NMAP to get MAC addresses remotely (ie when there is a router/layer 3 device between yourself and the scanned target)? If truly possible, that could be a powerful tool for determining the hardware platform of a remote device.

In the case of the Netgear WGR614, based upon the specs the wireless and wired interfaces are bridged together, not routed. Thats why MACs are visible in this case.