This (airplay) is probably the killer feature for the appleTV, and to a certain extent the airport express.
Being able to switch from watching/listening to content on my iOS device to listening via my various systems around the house is extremely compelling for me. Using the existing remote app is clunky at best on iOS devices, so this looks to be a real leap forward in usability.
The part that I'm curious about is whether that will extend to streaming content via the web, or just content that's local (sync'd) to the iOS device. For example, if I'm watching a video on some site like Lifehacker on my iPad, in theory airplay should allow me to stream that to one of my aTV's.
it will be very interesting to see how this plays out. I know a lot of people are bent out of shape over things like no 1080p and tv show rentals vs purchasing, etc... But for me the main use case has to do with how these devices work together (desktop/laptop, airport express, aTV, iPad/iPhone/iTouch). This looks like a great step in the right direction.