This appears to apply to wifi as well. There's been an open bug in OSX for years involving OSX assgining its own DHCP lease when it fails to negotiate one with the router. This results in the dreaded "Self-assigned IP" message, which is nigh-on impossible to rid yourself of short of voodoo dolls and wifi dances.
Getting a bit off-topic here, but that's not a bug: it's assigning itself a valid zeroconf* address because the DHCP server is not responding.
* http://en.wikipedia.org/wiki/Zero_configuration_networking#A...