Okay... Feeling dumb. I'm on 10.6.5, running iTunes 10.1 (54) and I don't see the Airplay button in the bottom right corner of iTunes.
I'm running 4.2.1 on my iPhone and the latest version of the Apple TV software, as well. From my iPhone, I can push video or audio to the Apple TV, but not from iTunes. The button just isn't there and I can't find anything in the Menu Bar.
What am I missing?