Rorate Caeli has the text of the Holy Father's official declaration at the consistory.
The most relevant section:
However, in today’s world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith, in order to govern the bark of Saint Peter and proclaim the Gospel, both strength of mind and body are necessary, strength which in the last few months, has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me. For this reason, and well aware of the seriousness of this act, with full freedom I declare that I renounce the ministry of Bishop of Rome, Successor of Saint Peter, entrusted to me by the Cardinals on 19 April 2005, in such a way, that as from 28 February 2013, at 20:00 hours, the See of Rome, the See of Saint Peter, will be vacant and a Conclave to elect the new Supreme Pontiff will have to be convoked by those whose competence it is.
An update from the Vatican spokesman says that Benedict will finish up his work and then retire to the cloister within the Vatican walls.
Father Z's blog is not loading, I assume from heavy traffic. It's all over the news, so you'll have no trouble finding stuff, though beware of rumor and gossip as I doubt anyone knows anything beyond what's stated above.
Pray and fast, brethren.
Update #1: The last pope to resign, Gregory XII.
Update #2: Father Z's blog is back up. He has several new posts, including this one summing up his thoughts so far. Worth a read.
Update #3: Let the post-pontificate comparisons to John Paul II begin! Note the inaccuracy about where John Paul got his papal name. It's my understanding it took it in honor of his predecessor, not for John XXIII and Paul VI.