From the official Vatican Web site:
January 22nd, 1506, is the official date of birth of the Pontifical Swiss Guard, because on that day, towards the evening, a group of one hundred and fifty Swiss soldiers commanded by Captain Kasparvon Silenen, of Canton Uri, passed through the Porta del Popolo and entered for the first time the Vatican, where they were blessed by Pope Julius II.
From the Associated Press via CBS News:
"Thank you for your service of 500 years!" Benedict told the guards in a special blessing, to applause from tourists and the faithful also gathered for his traditional Sunday greeting.
From now through June, the Vatican will commemerate the 500th anniversary with many special events, including a re-enactment of the march from Switzerland to Rome of the first 150 Swiss mercenaries. From all of us at the Pope Blog, a very special thank you goes out to all Swiss Guardsmen for a fine 500 years of service.