OK, the beast has arrived courtesy of UPS and I've been playing with it
Some questions...
1. I can only use Apple TV (playing a movie) and the Airport Express (playing music) simultaneously if I start the music first, if I start a movie first iTunes then can't send to the Airport- I get an "unknown error" when it tries to find it. So far that seems to be true even when using two computers... it's not a disaster, but it's not what I was expecting.
2. It seems to only work by "pulling" data from iTunes, it doesn't seem that you can use iTunes to "send" a movie to the the TV (a music track is OK). Bit of a bummer if you can't find the remote, which looks far to easy to mislay

.
3. I can't see any way to put it on to standby without going through the menus. I've no environmental objection to leaving it on 24/7, but I'd marginally prefer not to.
In all cases, is that working as intended or have I missed something?
It won't respond to the remote that came with the iMac either (no, it's not paired), but there's some other oddities with the remotes that I need to investigate before asking any definite questions.