The Apple TV has received a major software update, adding several important new features, including iTunes Radio in the U.S., as well as the iTunes Music Store. iTunes Radio works like it does on iOS devices and desktop computers, allowing you to listen to listen either to the pre-existing stations or to those you make yourself.
One of the more appealing aspect of iTunes Radio, at least from the standpoint of the record labels, is that you can buy from iTunes Music Store the songs you stream through iTunes Radio, functioning on the Apple TV as it does on every other iOS device. Previously, you could only use your Apple TV to listen to your previously-purchased music.
There are other important changes with the new software, including AirPlay from iCloud. This allows you to stream your content through an Apple TV without being logged into your iTunes account. Using an iOS device, you can direct the Apple TV to stream your content from your Account. You can now view photos and videos that have been added to your iCloud shared photo stream. You can also now sync your podcasts across your Apple TV and iOS devices. This is especially useful if you subscibe to a lot of video podcasts through iTunes.
To download the update via the Apple TV, go into Settings > Update Software, and consent to the update.
Update: We're getting reports of the Apple TV 6.0 update bricking a number of Apple TV (2012/1080p). If you're not comfortable connecting your Apple TV to iTunes on Windows or Mac and restoring/updating manually, you might want to hold off.
Update 2: Apple TV 6.0 Software is no longer showing up. Looks like it might have been pulled. We'll keep updating as more information becomes available.