Hmmm, I'm confused by your last statement.
The IP address and the MAC address are not related to each other. Each device has a preprogrammed, unique MAC address that is used. For what I've shown in the sketch, the arduino is trying to use a dedicated IP (192.168.1.2).
DHCP sit's on top of the TCP/IP stack as an application. This requires an additional set of handshaking that has to happen. I remember that somebody created a library for DHCP using the ethershield. I'll have to go dig that up as reference.