Apple has released a minor firmware update for Apple TV, taking the unit from version 2.3 to 2.3.1. The changelog has yet to appear on Apple's site, but Apple Insider users are discovering just very minor feature changes, such as the inclusion of a built-in bandwidth monitor. (Unfortunately, we got the error message pictured when we tried to use it.)
Otherwise, the only big achievement of the 2.3.1 update is to cripple the Boxee installations that some enterprising users have added to their Apple TV. (Boxee, a popular software package that allows easy access to Web-based videos, is an unauthorized hack that's not supported by Apple.)
Bottom line: if you're running Boxee on your Apple TV, you'll want to turn off automatic updates. You can be content that (with this update, anyway) you don't appear to be missing much.