Changing my router's wifi channel has fixed this for me. I was on channel 7 and the airplay icon would often disappear from my iPhone. As soon as I changed to channel 3, the icon is always there.
Changing back to channel 7, the icon disappears again.
I can't promise this will work for everyone but it's an easy solution to try.
Thanks DrWevil. that worked for me. The latest update had the 'mirroring' turned off. On the iPhone 5 all you need to do is double click on the home button, swipe UP to get the music control screen, select 'airplay', then select Apple TV and turn on 'Mirroring'. This will probably happen every time there is an update.