I'm using 13 Alpha 3 and whenever I minimize XBMC as soon as the built in screensaver turns on it causes XBMC to become unresponsive and the only way to close it is to force close via task manager. It does this every single time. The only way to stop XBMC from freezing when minimized is to turn off the screensaver.
http://xbmclogs.com/show.php?id=17709
http://xbmclogs.com/show.php?id=17709