When the pope says he's going to step down due to his deteriorating strength of mind and body, it's hard to say much more than that about it with any certainty. That said, I found Rose Berger's post pretty thoughtful:

As a cradle Catholic who loves the church enough to fight with her when she fails to live up to her gospel call, the words “the See of Saint Peter, will be vacant” are words that strike a dark loneliness in my stomach and soul. . . . 

Which of the following are likely true?

• He never really wanted the job and hoped to retire after the death of John Paul II, but took it up anyway and it turned out to be one of the hardest 8 years in recent Church history.
• He’s frail, weak, extremely tired; like Jesus, he’s fallen the second time and needs someone else to take up the cross.
• There are younger men who he feels can better respond to the Church and world’s contemporary needs.
• The pedophilia scandal continues to be a mark of shame on the Catholic church and while his heart is broken over it he does not have the courage or calling to change the system.
• The Catholic Church is on the verge of a major shift to reclaim the full spirit of the Second Vatican Council and the pressure within has become such that he needs to step out of the way.

I would say that “all of the above” are true. And in mid-March, when cardinals from around the globe arrive in Rome, the world will see for the first time in living memory a new pope elected while the previous pope looks on.