Satcomer - just a note, this seems at first to be a relatively unimportant vulnerability, but it might actually not be.
As you noted, an attacker does need to control one computer on your local network, and could then use it to take over any Macs you own that use DHCP. So, there would already have to be one security breach just to get you access to this. Still, it's a serious enough problem for a couple of reasons.
For one thing, if you use airport, it could be pretty easy to insert a computer onto your local network. Could do it from across the street. If you go to an Internet cafe with wireless access, you'd probably use DHCP to get the right IP settings. So, an attacker could take their laptop to an Internet cafe, and take over people's powerbooks.
For another thing, if you have a Windows box and some Macs, someone might take over the Windows box (hardly unlikely) and use that as a vehicle to take the Macs.
And, in a large organization or lab setting, it's relatively easy to get temporary control of one computer - all you have to do is boot it off a CD. In that situation though, the really important files are likely to be on a server in a locked server room somewhere, maybe encrypted, and you'd need the right passwords to get at them. The workstation itself is not that valuable a target. But, if you set up a Linux boot CD that runs a DHCP server, you could cause any Macs in that lab or workplace to send their passwords to you, not the regular authentication server. Then you'd have the real information you were after...
Of course, it's not as bad as the sort of vulnerabilities that are found in Windows once every month or so, but it's still pretty serious