Today Is 101010: The Ultimate Answer to the Ultimate Question

Today is October 10, 2010. 10/10/10. In binary, that's 42. And 42 is The Answer to the Ultimate Question of Life, the Universe and Everything. Or at least, that's what Douglas Adams says.

Many people wonder what Adams exactly meant by 42, the answer given by the supercomputer Deep Thought in The Hitchhiker's Guide to the Galaxy. Why did Adams pick that number? Is there a connection to something the world doesn't know about? Is the CIA and the MI6 involved in all this? Real aliens, perhaps?
On November 3, 1993, he gave an answer on alt.fan.douglas-adams:

The answer to this is very simple. It was a joke. It had to be a number, an ordinary, smallish number, and I chose that one. Binary representations, base thirteen, Tibetan monks are all complete nonsense. I sat at my desk, stared into the garden and thought '42 will do'. I typed it out. End of story.

Later, talking to BBC Radio 4 Iain Johnstone, he explained that the number was chosen by none other than John Cleese as the punch line for one of his skits. The famed Python thought it was a funny number, and Adams borrowed it for his book, turning it into a recurring integer through all his work.
But that comment wasn't the end of the mystery. Stephen Fry—a friend of Adams—also jumped into the debate, claiming that the latter explained to him why it was 42. Fry will not reveal the secret, but he says it is "fascinating, extraordinary and, when you think hard about it, completely obvious."

Whatever it is, it sure has had a deep impact in geeklore. One example: The Allen Telescope Array—the radio telescopes system erected by Microsoft's Paul Allen for the SETI program—has 42 dishes in honor of Adams. And in Lost, 42 is the last number in the sequence that has to be entered on The Swan's computer, which is also the sequence picked by Hurley for his winning lottery ticket, and Kwon's number in the cave. In a Lostpedia interview, one of the show's producers confirmed that this was indeed a homage to The Hitchhiker's Guide to the Galaxy.
This date only repeats itself every hundred years. The next one will be in 2110. [Thanks Paul Cohen!]