On October 30, 2007, Bishop-designate Callahan was appointed auxiliary bishop of Milwaukee. He was the first Conventual Franciscan to be named a bishop in the United States.(via Annysa Johnson at JSOnline)
On April 20, 2009, the College of Consultors elected him to serve as the administrator of the Archdiocese of Milwaukee until Archbishop-designate Listecki was installed as archbishop of Milwaukee on January 4, 2010.
Update: Brian Olszewski reports at the Milwaukee Catholic Herald.