We have Avaya IP phones which can be managed well with a dhcpd (ver3+) server, but they need to have a couple of special settings on the dhcp server to work automagically.
In the /etc/dhcp3/dhcpd.conf file of a dhcp server that *does* support Avaya IP phones, the line says:
option option-176 "L2Q=1,L2QVLAN=20";
The option number 176 is just an example; it would be great to support arbitrary dhcp configuration commands.