On a MBP (OS X 10.6.2) I've got the wired ethernet and wireless airport configurations set up to use the same static IP to my router (e.g. as described at http://forums.macrumors.com/archive/index.php/t-708685.html). I've also got the Service Order set up so that the wired connection is above Airport (e.g. as described at http://superuser.com/questions/83943/mac-os-x-automatically-turn-off-airport-if-ethernet-cable-is-plugged-in/83952#83952).
This seems to "work": an open ssh connection to a remote location stays up as I plug in and unplug the ethernet cable. However, since the Airport card stays live (and connected to the router) regardless of whether the wired connection is present, how can I tell that the wired interface is actually the "active" connection when it is available? Since the IP address is the same for both, is this a guarantee that the wireless interface cannot be used? Is there some other way to tell if this interface is in use?
PS. I'm going to post a (partial) answer as soon as I post this, as the act of writing this up and checking all my facts lead me to some insights. (The more interesting and unanswered part is in bold above.)