Have found a quick way to stream my mac through Airplay to the Apple TV.
It's called Airparrot, it's free if you accept a time limit on each streaming, but there is a cost if you want the full version. I don't necessarily need the streaming so I've only got the free version so far but it's really handy and is nowhere near as laggy as I thought it might be.
I know it is something that will come natively with Mountain Lion but thought I'd mention it in case any of you can't wait until Mountain Lion.
I'm absolutely loving the Apple TV so far. In typical Apple style it's not something I ever thought I'd need until I got it, and now I already want one for each room.