Don't know if it's related to PHP5RC2 or not?
Try sticking all this at the top of admin/index.php and see if it takes:
PHP Code:
// prevent caching code from php.net
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT"); // Date in the past
header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT"); // always modified
header("Cache-Control: no-store, no-cache, must-revalidate"); // HTTP/1.1
header("Cache-Control: post-check=0, pre-check=0", false); // HTTP/1.1
header("Pragma: no-cache"); // HTTP/1.0