You could use a Nyrius Wireless AV Sender. If you're looking for a small, slick unit to get your presentations displayed while visiting clients it probably isn't for you. If you want to get content from your Macbook Air onto your wall-mounted display at home without stringing a long HDMI cable across the room it might be just what you want.
An alternative - look at this youtube video showing how to do it with an intermediate iPad, maybe not so practical. It uses an app called iDisplay for $4.99. In that it seems to be doing the same kind of function as Air Display which I use and would recommend over anything else.
Second alternative - AirFlick from Erica Sadun - allows you to make your Mac an AirPlay source so you can mirror its display to an AppleTV, just as if it was an iPad. This won't be as easy as we might expect of Mac software, Erica is an awesome hacker but doesn't employ a team of graphic and web designers to make beautiful lick able software.