Ars on their new 24-inch Apple LED Cinema Display:
The real nicety of the display is that Mac OS X knows when you have attached the display to use its integrated devices. That is, when you’ve hooked it all up, it will use the iSight in the display instead of the notebook’s, and it will use the USB audio on the display and disable the output on the notebook. That is, until you plug a set of headphones into the port on the notebook, at which time the display’s speakers will disable and route the audio directly to your ears automatically.
It’s the little things, you know?