42

From the BBC:

Tragically, Douglas Adams died in 2001. So what does Stephen Fry, a close friend, voice of the audiobook, and possibly one of the most intelligent admirers of The Hitchhiker's Guide think?

"Of course, it would be unfair for me to comment," he confides. "Douglas told me in the strictest confidence exactly why 42. The answer is fascinating, extraordinary and, when you think hard about it, completely obvious. Nonetheless amazing for that.

"Remarkable really. But sadly I cannot share it with anyone and the secret must go with me to the grave. Pity, because it explains so much beyond the books. It really does explain the secret of life, the universe, and everything."

http://news.bbc.co.uk/2/hi/uk_news/
magazine/7283155.stm