Fotiman - 1:24 pm on May 30, 2012 (gmt 0)
Welcome to WebmasterWorld!
I suspect, though, that the page that gets loaded in the iframe should have some HTTP headers set to prevent it from being cached. For example:
header("Cache-Control: no-cache, must-revalidate"); // HTTP/1.1
header("Expires: Sat, 26 Jul 1997 05:00:00 GMT"); // Date in the past
Hope that helps.