To date, there are no 3rd party software implementations of AirPlay. The spec itself is fully patented (though most of these patents relate to the hardware aspects of the system) and is essentially a proprietary standard unique to Apple (though several hardware vendors have announced hardware that will use the standard - XtremeMac, Denon). The current analysis of the protocol itself seems to indicate that it is a further evolution of the previous AirTunes protocol (which itself was built on top of Bonjour), so you are likely to see software implementations appear after developers have had more time to experiment with the protocol.
So far I haven't found a good all-digital solution. I typically use an audio splitter at the computer, and a whole house FM transmitter plus speakers so I hear the sound locally and through FM radios I place wherever I want the sound.
Since everything is transmitted real time, there's no sync delay as you move from room to room, and the transmitter I have is strong enough to receive using a boombox out in the yard while I'm working outside, without risking any expensive equipment in case it gets kicked around the yard or rained on a little bit.
I use a modified Belkin Tunecast transmitter. I disassembled it, and added a length of wire to the antenna connection to stretch the distance it transmits a little. There are some legal ramifications to this depending on the country you live in. If you want a more professional solution, there are a few places that sell whole-house FM transmitters fairly inexpensively.
You could also connect the transmitter to the airplay receiver, entertainment center, iPad, Android phone, windows PCs, or any other audio device, and the receivers are cheap - you can get an FM radio from the dollar store for a dollar, and amplified speakers for a dollar, so expanding the system is cheap. It's very flexible.
The biggest drawback is quality. It's not bad, but you can't expect even CD quality from it, nevermind high quality audio. It's ok for music and voice, but audiophiles will be offended. Airplay is CD quality, and the difference would be noticeable.
Best Answer
If you want to watch Netflix on your Mac, the best way to go is just use Netflix.com (you'll need the Silverlight plugin).
AppleTV doesn't have any capabilities to send streams (ala AirPlay), just receive them from other devices. The reason for this is effectively that the Apple TV doesn't really hold any content. It sources it all from other places, all of which can be accessed from your Mac (i.e. iTunes, Netflix, Hulu, etc.).