Comment 5 for bug 351378

I believe this is a missing feature of ISC's dhclient. I can't find any mention in the source of supporting virtual/alias interfaces outside of the alias declaration in dhclient.conf(5) that allows you to specify a static address to be applied to an alias interface in addition to the address acquired by DHCP.

I did test and confirm that dhcpcd=1:3.2.3-1.1 does work on alias interfaces on intrepid.