I'm currently configuring a laptop for someone; given the likely usage 
environment, setting up dhclient is going to be painful.  I tend to 
think that the model we're using is wrong.  Windows XP, at least as 
configured by IBM, seems to do it properly.

The machine will be used in both wireless (built-in am0 interface) and 
wired mode.  For fairly obvious reasons, it's generally better to use 
the wired interface if it's available.

Right now, dhclient cycles through various media options for devices 
listed in dhclient.conf.  This is especially important for 802.11 
devices, where there might be several combinations of keys.  In a 
fairly strong sense, dhclient shouldn't bother trying the DHCP protocol 
unless it sees RF carrier on a wireless interface; similarly, it 
shouldn't try DHCP on a wired interface if the cable is disconnected.

We almost have the right primitives.  ifwatchd notices when my Ethernet 
cable is connected or disconnected, though it doesn't do the same for 
wi0.  It's possible in theory (and, I'm told, on -current in practice, 
though I haven't tried it yet) to add and delete devices from dhclient.

I suggest the following strategy for such environments:

	Have an 802.11d (some would call it wifid, or even WiFid) that
	tries various combinations of nwid and nwkey, until it sees a 
	carrier.  Loop as needed, because 802.11 carrier can go away...
	There may need to be some priority metrics in the config file 
	for this daemon

	Have an enhanced (to use a config file instead of command line
	arguments) ifwatchd (or a wrapper for it) notice the coming and
	going of carrier on all specified interfaces, wired and 
	wireless.  When carrier appears, use omapi to add those 
	interfaces to dhclient; similarly, delete interfaces when
	carrier disappears.

	Enhance dhclient to know about interface priorities -- run
	DHCP on an interface if and only if there are no higher-priority
	interfaces that respond properly to the protocol.  If a higher-
	priority interface should suddenly appear or suddenly respond,
	unconfigure the IP address from the lower-priority interface.
	(That one is a bit tricky -- you may have open connections.
	At least, the default route should be taken away from the
	low-priority interface.)  The essential point is that dhclient
	should -- or at least could -- get out of the carrier-finding
	game on wireless nets, and should become cognizant of carrier
	on all interfaces.

Does this sound like a reasonable approach?