From TUAW, AppleTV update 2.3 released:

• AirTunes Streaming from Apple TV - Music can be streamed via AirTunes to Airport Express speakers or other Apple TVs in your house.
  
• Third-party Remote Controls - Apple TV can now learn other remote controls and use them in addition to the Apple Remote.
  
• Playlists - Playlists in iTunes that contain Movies, TV Shows, Podcasts, and Music Videos can now be seen on Apple TV.
  
• Music Volume Control - Support for volume control in Music.

Cool! I love my AppleTV, and will probably wind up using all of these features. In fact, I was bemoaning the lack of music volume control jut last weekend while I sat on my deck. Hopefully this feature will make it into the Remote iPhone app soon!