Apple has updated the Apple TV software to version 2.3, adding a few refinements to the company's "digital living room" appliance.
In an earlier update, Apple added the capability to use the Apple TV as an AirTunes speaker: you can start playing music from your Mac or Windows computer in one room, and have it play in the living room via the Apple TV, in another room via an AirPort Express, and so on. With the 2.3 update, the Apple TV can now be the music source, streaming to AirPort Express units or other Apple TVs on your home network.
If you own a third-party remote control (such as a Harmony universal remote), you can make the Apple TV learn its settings (go to Settings > General > Remotes > Learn Remote). The Apple TV also now recognizes iTunes playlists that contain movies, TV shows, music videos, and podcasts. (In a related change, I see that you can now play music videos back-to-back, although I'm not sure if this capability is specific to the 2.3 release; under the Apple TV Software 2.0, you could watch only one music video at a time.)
Finally, the Apple TV gains a feature that's been missing from day one: you can control volume (in music only) using the Apple remote.
The update is available only from the Apple TV itself: go to Settings > General > Update Software.