Apple has released a couple of updates: one for iTunes, bringing the latest software version to 9.0.2, and the other for Apple TV, which updates the software to version 3.0. These updates were released together because the update to iTunes adds support for the new Apple TV software. In addition to support and stability, a few new and requested features have been added to both of these updates.
iTunes 9.0.2 -- This update adds support for Apple TV version 3.0, and also adds an option to have a dark background when viewing songs and albums in "Grid" view, a feature of previous versions that was much missed in iTunes 9. The update should be available through Software Update.
Apple TV 3.0 -- This update adds a redesigned menu, and adds support for viewing iTunes extras and iTunes LP in full screen. You can also now listen to Genius mixes and internet radio via your TV and home theater systems. The update is available through the Apple TV settings in the "Update Software" section.