Typically with Mac OS X you have the volume control slider in the menu bar, and then if you want to make any other changes you would have to open System Preferences -> Sound and then make your changes….
I presume this is new in Snow Leopard, since I have not seen this documented previously, and I have never seen this before installing Snow Leopard….

If you option-Click on the Sound Volume control in the menu bar, an Output & Input Device menu will appear that will allow you to re-assign your active Output device & Input Devices.
A very welcome feature to those that will end up using it. Off the top of my head, anyone that needs to use headphones part of the time… Or podcasters, or….