All of you guys replying with instructions of how to sleep the machine rather than the display should be ashamed, read posts in future- you're not helping, this thread was incredibly irritating to read through and find my answer.
BetterTouchTool is the way to go for this it seems, but for my use case at least, 'sleep display' isn't appropriate, it renders your machine unusable on connected AirPlay devices, which is what I was looking for.
If you want to 'sleep display' your machine so that it isn't glowing whilst you're using AirPlay, this isn't going to cut it, the device becomes unresponsive- much like regular sleep it seems, I'm not entire sure what the difference is.
Since I wanted to just stop my MacBook Air glowing whilst I'm AirPlaying, I used BetterTouchTool to bind 16x brightness down and 16x keyboard lights down actions to a single key (I chose the key beneath ESC) and similarly, shift + that key to do the opposite. Effectively making my screen dim to black, and keyboard lights turn off whilst still leaving AirPlay working fine so I can watch streams on my AppleTV 🙂
Just to confirm it looks like this: http://cl.ly/image/0d0g0Q0A3C3L
Works great! 🙂