Mac OS X StartupItems
DNS StartupItem - description forthcoming.
Multiple IP StartupItem - description also forthcoming.
(All of this is for Mac OS X 10.1 and earlier... in 10.2 you can do multihoming easily with either `networksetup` or the Network preferences pane and DNS will start automagically is you turn it on from "Server Settings.app".)
These are really just things you might find handy, they aren't guaranteed or anything, just something to play with, really.
As with any of this sort of stuff, if you have the source (and since these are shell scripts you, ehm, have the source), give a read through, to at least make sure it isn't going to hose your particular system.
software@supertart.com - QI Software.