I use a Plantronics wired headset for phone calls with my Droid X, which works great. When trying to shoot video, while this headset is plugged in, the Droid insists on getting the audio from the onboard mic(s). The headset mic seems to be disabled while in video mode.
So I assume from this test that an external mic scenario for video just isn't going to work. Unless maybe a software change can allow selection of audio source.