I'm posting this for future reference. If you invoke a PageFunction and add an event handler to the PageFunction's Return event, the event handler must be an instance method of the calling page. No other method will do. If the event handler doesn't meet this criteria, you'll see the FatalExecutionEngineError thrown when you return from the PageFunction. Sorry, no separation of concerns allowed.
More details in this forum thread: http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=1975763&SiteID=1.
