Actually, if you want to sit down and analyze it, I suppose I do use it in that sense. When referring to the actual thing you're seeing and/or are invovled in, it's "theatre." The venue is the "theater."
"I'm going the theatre tonight!" or... "The theater is on 46th..."
*starts off singing in her own little world*
"You say 'aa-fter' and I say 'ah-fter.' You say 'laa-fter' and I say 'lah-fter.' 'Aa-fter,' 'ah-fter,' 'laa-fter,' 'lah-fter.' Let's call the whole thing off!!"
"You! You are the worst thing to happen to musical theatre since Andrew Lloyd Webber! And you, well, I just plain don't like you."
~Stewart Gilligan Griffin