Keep in mind that MAC addresses will only be available for machines on the same physical network segment as you, so unless this is just intended for use on a single LAN that'd be heading down the wrong track.

A much better solution would be to use SSL/TLS certificates to strongly authenticate clients.

Update: Also note that it's not impossible to spoof another host's MAC given the right OS and a cooperative NIC. Granted they'd have to knock the real host off the network or it'd probably send packets with RST set when it sees traffic for TCP connections it knows nothing about, but traffic from a given MAC doesn't guarantee it's from that NIC.