He's like a porn star - when you are that famous you can pick any name you want ;-)

Just kidding. Here's what I found:
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One of the most interesting aspects of the papal election process is the tradition where the newly-elected pope takes a new name. Frequently, the selection of a name signifies something or signals a direction the new pope might want his papacy to take.

John II (533) was the first pope to change his name because his original name was “Mercury,” the name of a Roman god. He thought it most appropriate to change his name for that reason. Another interesting name change occurred at the election of a new pope in 983. He took the name John XIV because his given name was Peter. Out of respect for the first pope, he believed he should not become Peter II. Since the 11th century the practice of changing one’s name has been common. Marcellus II (elected in 1555) was the last pope to keep his original name.

On Oct. 28, 1958, 77-year-old Angelo Giuseppe Roncalli became Pope John XXIII. His choice of a name carried a message. Upon his election, when asked what name he wanted to choose, he chose John XXIII.

This at first may have shocked the other cardinals because the same name was assumed by an unscrupulous man in the middle of a divisive 15th century controversy — a controversy that involved three men simultaneously claiming to be the pope.

Mr. Roncalli said, however, that John had been his father’s name and also the name of the church where he had been baptized. Furthermore, perhaps with a hint of humor from the near octogenarian, he said the name was the most frequently used by popes and that most had short tenures.

In 1978, Albino Luciani became pope and created a new papal name, John Paul I, showing deference to his two immediate predecessors (John XXIII and Paul VI). Shockingly, he died only a month into his pontificate. His successor, Karol Jozef Wojtyla from Poland, took the name John Paul II to show respect to his stricken processor.

In a few days, when a new pope is selected from among the cardinals, pay attention to his selection of a name. Admittedly, the next pope has huge shoes to fill. Will he take the name John Paul III? It is anyone’s guess. The new name might give a hint about the direction the new pope wants to take the Roman Catholic Church in the future.
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and
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There's no requirement that a newly elected pope change his name, but it's a practice that has been adhered to for more than 1,000 years. One papal historian traces it to the Book of Genesis, when Abram took the name Abraham. Popes make their name selection known immediately after election; their choice often honors a previous pope or a famous saint.
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"How bad could it be?" - SimpleS

FWIW, Prince William of England can choose one of his given names (William Arthur Philip Louis) or any other name if/when he becomes King.

Judging by the quickness of the name choice, I wonder if every cardinal in the conclave had their Pope name picked out beforehand.

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"Go yell at an M&M"

I'm sure it's something they think about beforehand.

Thanks for the info. Interesting stuff. Slowtwitch is much better than goggle.

If your name was Ratzinger, wouldn't you want to change it? :)

"Let me see ... I could use my real name and call myself Pope Joe, or I could call myself John Paul III ... wait JP-3??? JP-3??? Isn't that some kind of jet fuel??? No. I'm just gonna call myself Pope Benedict XVI, that sounds fine, plus it has all the Roman numerals in it, that should make the Italians happy ..."

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"What's your claim?" - Ben Gravy
"Your best work is the work you're excited about" - Rick Rubin