Alfred Muller, a 58-year-old suburban mayor, pleaded guilty Friday to sexually abusing a 14-year-old boy in Washington's National Cathedral.  Muller is a longtime mayor of Friendship Heights, Maryland and a volunteer usher at the cathedral for 35 years.  He encountered the teen in a bathroom on January 28th and reached over and touched him as they stood at adjacent urinals.  The boy identified Muller from a photo in People Magazine, which carried a story about Friendship Heights' smoking ban.  The boy was at the cathedral on a field trip.  According to the Associated Press, in exchange for Muller's admission to one count of sexual abuse of a minor and registering as a sex offender, prosecutors agreed not to oppose a sentence of probation on the retired physician and teacher at Georgetown University medical school.  If the judge accepts the plea to a misdemeanor on May 11th, Muller's sentencing day, the mayor might not serve any time in prison.  The maximum penalty is 180 days in jail and a $1,000 fine.