The Slingbox will stream to the Apple TV in HD. The SlingPlayer app is designed to be in Auto when using with the Apple TV. Which means it will adjust SD & HD to what the internet speeds are currently at. To stream in HD you must have a 2.0 mbps upload at home and 2.0 mbps download at the remote location. If you need to test the speeds you can go to Speedtest.net.
Here is an article to help explain this information.
Sling Media Forum Moderation Team