I'm not sure the charges can stick, but since Donald Rumsfeld has resigned, he also loses immunity protections. However, I can't see the case proceeding very far. Still, a report in Time magazine says a case is moving through the legal system in Germany:
"Just days after his resignation, Defense Secretary Donald Rumsfeld is about to face more repercussions for his involvement in the troubled wars in Iraq and Afghanistan. New legal documents, to be filed next week with Germany's top prosecutor, will seek a criminal investigation and prosecution of Rumsfeld, along with Attorney General Alberto Gonzales, former CIA director George Tenet and other senior U.S. civilian and military officers, for their alleged roles in abuses committed at Iraq's Abu Ghraib prison and at the U.S. detention facility at Guantanamo Bay, Cuba."