Apple has pushed out the iOS 5.1 update for the 2nd and 3rd generation Apple TVs to include support for shared Photo Streams and more (as noted by 9to5Mac). Here’s the full changelog:

• Shared Photo Streams – Accept invitations for Shared Photo Streams, browse photos and comments, and receive notifications of new content.
• AirPlay – Send audio content from Apple TV to AirPlay-enabled speakers and devices (including AirPort Express and other Apple TVs). Also includes the ability to require an onscreen code to use AirPlay with your Apple TV.
• iTunes account switching – Save multiple iTunes accounts and switch quickly between them.
• Trailers – Search movie trailers. In the United States, see show times for local theaters.
• Screen savers – New Cascade, Shrinking Tiles, and Sliding Panels screen savers.
• Main menu – Reorder icons on the second page by holding down the select button on the remote.
• Subtitles – SDH support for the deaf and hard-of-hearing as well as improvements to viewing and selecting subtitles.
• Network configuration – Support for setting up advanced network options using configuration profiles. See http://support.apple.com/kb/HT5437 for more information.
• Stability and performance – Includes general performance and stability improvements.

You can download the standalone updates here for the 2nd generation Apple TV and 3rd generation Apple TV here. Or, just navigate to Settings — General — Update Software on your Apple TV.