One word marissa...ALCOHOL! You have to watch the episode for yourself but basically they were both drunk and George was telling Izzie that Callie was jealous of her and one thing led to another and the next thing you know Izzie is waking up naked next to George. I thought it was all a bit ridiculous. :)

yeah just watch the ep its here on the site you dont c anything jst izzie waking up and seein george naked beside her was funny!