Have a 2nd generation Apple TV hooked up. Am able to AirPlay with my iPhone, but not with the Macbook. Not sure if this is even possible, but Googling around seems to return some chitchat about it, but nothing concrete.
Considering the Airplay web page displays a Mac sending the signal, it sould be perfectly possible. As per this answer on the MacRumors forums, it seems to require iTunes 10.1 though:
Airplay without video was called AirTunes. iTunes up to 10.0 only supported that.
You might also want to take a look at Erica Sadun's Air Play Utilities:
As pointed out by Apple on their AirPlay Support website, any 2011 MacBook is capable of supporting AirPlay mirroring and can mirror to an Apple TV.
Here are the supported devices for AirPlay mirroring.
On newer Macs, it is very simple to mirror your screen onto an AirPlay-compatible device such as an Apple TV. There is an AirPlay status menu on the Menu Bar.
If you are using an earlier Mac, you are not doomed however; while it is not officially supported, there is software out there that will let you bypass the compatibility issue, and to be quite honest, many of these apps work very well.
Here are the main apps that let you do just that.
I have personally tested out AirParrot and was surprised at how well it works.