I'd say 10:30 is about the latest you can stay in Epcot after close. (a little later than that during the holidays when Illuminations is at 9:30 because of the Candlelight Processional) The only time we were 'escorted' out of World Showcase was in October 2004, the night that there was a total lunar eclipse. I think it peaked at about 10:50pm. The Epcot managers were on their Segways, doing the security sweep of the park. They came up to us, watched the eclipse peak with us and then asked us to leave! They had to radio to International Gateway to let them know to let us and another group of people who were taking pictures of the eclipse out. It was one of the coolest things we've done at WDW!
-Michelle