This past weekend John Piper spoke at The Cove in Asheville, North Carolina on Romans 12-13. The Cove is the Billy Graham training center which features various speakers and seminars throughout the year. If you've never been, the property itself is "a little bit of heaven on earth." Please check it out (I swear I get no kickback from this).
Anyway, Desiring God ministries now has the audio available—these are excellent sessions! http://www.desiringgod.org/Blog/740_more_concentrated_romans_audio/