Well, the answer is, sort of...
As of now, the iPhone SlingPlayer app can stream AUDIO to the AppleTV via AirPlay, but not video. The AirPlay icon appears right next to the volume slider.
As for the iPad SlingPlayer app, I currently see no way to stream either audio or video via AirPlay.
However...once Apple allows AirPlay video streaming capability for 3rd party apps in iOS 4.3 (to be released in a matter of weeks, if not days), it SHOULD be possible to stream the video as well. That is, IF Sling implements full AirPlay video support in its iOS apps right away (ahem, cough cough.....)