No. Just like streming a video to your iPad is harder on the battery than playing on stored on your iPad.
When the video is played via airplay on the ATV, it is buffered into the ATV's flash memory space. Then there is no more wifi transmission to stream anymore when it's done streaming to the ATV.
So yes, it's very efficient and does not drain battery. You save a significant amount of battery drain IF your video is native mp4/m4v encoded video/container. It uses hardware acceleration and dramatically improves battery life.
If you are airplaying something else other than the native encoded formats, then it will tax your cpu and very much and you will see a big drop in battery life. but with airplay, it does not eat up your battery like darngooddesign says it does.
PS - what's cool is that you can use the remote on the ATV to control the video - it is reflected on the iPad as well. Just turn off the ipad's screen when it's airplayed out to your atv.
It sounds like its more efficient than just streaming, but its still hitting your battery hard until its finished buffering to the ATV. How long does a 2 hour movie take to stream from your iPad to the ATV?