William Landreth, the computer whiz dubbed "the Cracker" for his ability to electronically break into various closely guarded computer networks, has acknowledged that he was in violation of his probation by vanishing last year. Landreth, 22, of Poway admitted to U.S. District Judge Rudi Brewster in San Diego that he left the area without permission, failed to report to probation officials and changed his address without approval. Brewster ordered Landreth to complete a 90-day diagnostic study at Metropolitan Correctional Center in San Diego. Sentencing was set for Oct. 13. Landreth was arrested June 26 in McMinnville, Ore., where he had been living as a transient. When he was 18, Landreth pleaded guilty to wire fraud for breaking into an electronic mail network in Vienna, Va. He was placed on three years' probation and fined $87 but given no jail time. He remains in jail without bail.