i was also affected by the safe mode restrictions
fixed it by the following:
added to /config.php
define('WP_TEMP_DIR', '/my/writable/dir');
removed trailingslashit() from /wp-admin/includes/file.php
line 75
now looks like this:
function get_temp_dir() {
if ( defined('WP_TEMP_DIR') )
return $WP_TEMP_DIR;
maybe code needs to be fixed there in the next update