Inside OS X 10.8 Mountain Lion GM: AirPlay Mirroring
Managing multiple displays with AirPlay Mirroring
But wait, that's not all. You can still initiate AirPlay Mirroring while connected to a secondary display. Under Mountain Lion, connecting to an external monitor works virtually the same, apart from the fact that the system, by default, pics the resolution for you. Plugged directly to my HDTV via an HDMI cable, the Displays pane for each of the two displays picks a "best for display" resolution, although either can be scaled.
The built in display supplies an option to adjust brightness (above) while the HDTV pane (below) presents options to rotate the display in 90 degree increments, set a refresh rate, and configure underscan (which creates a black margin around the image on your television to prevent elements close to the edge of the screen from being clipped off; again, this is not usually necessary to set on modern HDTVs).
Even with an external display attached (mirrored or expanding the desktop), AirPlay Mirroring is still available. The pulldown AirPlay menu changes to reflect additional options, as you can now mirror the display with your external monitor independently from mirroring your display to however many Apple TV devices you have on your local network (below).
Because AirPlay Mirroring is a function of the system (dependent upon CPU video encoding hardware), it doesn't count against the number of directly-connected displays your video hardware can support.
New AirPlay features for older Macs
While Macs that are more than a year and a half old don't have the hardware to support wireless video mirroring, they can still make use of audio AirPlay features in Mountain Lion. Currently, under the Sound pane in System Preferences, the only Outputs available are hardware devices, such as internal speakers, headphones, or USB or line-out speakers (below).
Under Mountain Lion, even Macs that can't support AirPlay Mirroring can support wireless audio output (below) to designated Apple TV, AirPort Express units or other AirPlay compliant devices (including devices and receivers from Audyssey, B&W, Denon, iHome, JBL, Klipsch, Marantz, Philips, Pioneer, Samsung, Sony and Yamaha), allowing users to, for example, play games where the audio is sent out through a home stereo system.
However, unlike iTunes, the Sound Output controls don't allow you to stream audio to multiple sets of speakers at once. That means if you set system audio output to an AirPlay device, the local speakers will be muted.
Older Macs can of course still play videos to Apple TV connected screens via AirPlay within iTunes, and play audio from iTunes to multiple speakers through iTunes' "Multiple Speakers" feature (above).