OK, speculations are rampant, so I'll add mine to the soup:
From the moment he was elected, Pope Bneedict made it clear that he hadn't wanted to be Pope: he said that the announcement was "like a guillotine blade coming down."
Knowing that at his age he'd only have a few years, and knowing all the sotto-voce skulduggery that goes on in any big institution, including the Roman Curia, I'm betting he has been planning for his successor from Day One back in April 2005.
He's a patient man. He's planned 15 moves ahead, putting the chess pieces in place. Now, announcing RIGHT before Lent, he's basically forcing the Cardinals to wrap it up quick, because none of them wants to be out of their Diocese for the whole Holy Season, and certainly not for Easter.
Plus, with Benedict still being alive, they can each of them come in to consult with himn, personally, if they want, about who he has in mind as a good successor. And he can undermine the plotters (yes, I think there are plotters, too.) This --- I hope --- is the long-term architecture of the thing. I think Benedict's moves are wise, Spirit-guided, brilliant.
Then, Benedict says he is going to retire to a life of intense prayer in a monastic setting.
Something big coming up.
Really big.
People: pray, pray. pray.
I am pleased to see the Pope unnerve the media once again...
wonder what they are so afraid of... :)