Apple has released iTunes 10.5.1 beta 2 to developers along with an email (below) announcing the availability of iTunes Match for Apple TV. The update, available for download from the Developer Center, fixes several bugs and still doesn’t render iTunes Match to the public.
iTunes 10.5.1 beta 2 is now available and includes a number of important stability and performance improvements. iTunes Match is also now available for testing on Apple TV.
On your Apple TV, choose Music > Turn on iTunes Match.
Please remember to backup regularly and do not delete the music you add to iCloud from your computer.
Apple also released iOS 5.0.1 earlier today (which interestingly expires in a month!).