In a consistory today, February 11, Pope Benedict announced his resignation of the ministry of Bishop of Rome, Successor of Saint Peter, effective as of February 28, 2013, 8:00 PM, so that as of that time the See of Rome will be vacant and a Conclave to elect the new Supreme Pontiff shall need to be convoked by those whose competence it is. Apparently he does not intend to directly exercise papal authority or influence over the choice of the next pope, but wishes it to take place substantially as it would if he were to die.
An English translation of the Pope's statement and the audio of the Latin original is available on the website of the Radio Vaticana. Till now the reports and comments on the announcement have been pretty factual and have restrained from making any evaluation of the decision. Perhaps in part just to get the report out more quickly…
The pope gives as his reason for resignation, the recognition that his strength is no longer sufficient to adequately fulfill the Petrine ministry, showing a great humility and will for the good of the Church. It is not easy for anyone to voluntarily step down from a position of responsibility and honor, and few do it. The last one to resign the office in such a way Pope Celestine V in 1294. (Pope Gregory XII resigned in 1415 to settle the dispute about who was legitimate pope and the resulting schism.)
There is no particular reason to doubt that Pope Benedict's stated reason is his principal motivation. Still, he must surely also have considered what potential difference the precedent of a pope resigning in modern times could make, as well as the difference it could make to the choice of a new pope if the election is held while Benedict is still alive. One trusts in the guidance of the Holy Spirit, but the Holy Spirit also works through human deliberations, and so such considerations are quite valid. While Benedict may not exercise any authority over the election to be held after he has resigned the Petrine office, his counsel and thoughts may be influential.