When an SWF is dynamically removed from the DOM in IE8-9, the memory footprint of the SWF instance is not reclaimed until the browser tab is closed or refreshed. The memory leak occurs even if the SWF is embedded in an iframe and the iframe is removed from the DOM, so this bug affects both JWPlayer and YouTube embeds. 
http://code.google.com/p/swfobject/issues/detail?id=619
http://forums.adobe.com/message/4649985