Somehow a bunch of our RGS Agent Groups had values for both "DistributionGroupAddress" and "AgentsByUri" properties. This was causing the Response Group migration to stop dead in its tracks! Thankfully Russ over on the MS forums had a solution:
http://social.technet.microsoft.com/Forums/lync/en-US/d712b09c-606b-4a1e-abba-146948ba83d9/only-one-of-distributiongroupaddress-and-agentsbyuri-must-be-set-during-movecsrgsconfiguration?forum=lyncdeploy
http://social.technet.microsoft.com/Forums/lync/en-US/d712b09c-606b-4a1e-abba-146948ba83d9/only-one-of-distributiongroupaddress-and-agentsbyuri-must-be-set-during-movecsrgsconfiguration?forum=lyncdeploy