I noticed on my MBA 11" (1.8 i7) that iTunes takes a big toll on the battery. My battery showed 3:20 remaining. As soon as I opened iTunes (without playing music, just letting it idle in the background), my estimated time immediately went down to 2:15. Streaming a movie to AppleTV? Immediately down to 1:44 estimated time remaining. Yikes. After streaming the movie for a few minutes, I quit iTunes and the estimated time went up to 3:02.