For those who are running multiple cubes through a single instance of slush's proxy and your pool of choice has vardiff, beware that slush's proxy has a bug resulting in unsubmitted shares. 
Workaround: one instance of slush's proxy per cube. Each instance needs a unique --getwork-port.
More info is over at Dogie's setup thread: https://bitcointalk.org/index.php?topic=352658.msg4371031#msg4371031