Setting default microphone

[Daniel Laycock]

Hello list,
Long time listener here, but first time contributer.
I've recently upgraded to 10.04, but I've had a bit of a problem
trying to get my external usb headset to play nicely with my built-in
mic.  When I go to System/Preferences/Sound/Input and select the
external headset it works as expected.  However, when the device is
unplugged the default input device switches back to the internal mic.
Thus, every time the headset is plugged in I have to manually switch
the input device back the the external headset.  Is there any way I
can set it up such that the headset mic is the default input device
whenever it is plugged in?  Strangely, this is the behavior that the
sound output displays with the same headset.  When the headset is
plugged in I get sound there, and otherwise it's on the internal
speakers, with no need to change settings every time I plug it in.
Thanks in advance for the help,
Daniel