You've discovered one of the pitfalls of using non-standard markup and coding. Onresize is not a standard attribute to the body tag (see http://www.htmlhelp.com/reference/html40/html/body.html which summarizes the element well). While IE might have supported onresize in the past, it has no obligation to support it in the future.
That being said, I would double-check with MSDN and make sure that onresize is no longer a supported event in IE. Perhaps they got smart and supported it as an event on window instead of on body...
Good luck.
Jay