In mid-2002, researchers at Plymouth University in England actually put a working computer in a cage with six crested macaques. The monkeys proceeded to bash the machine with a rock, urinate on it, and type the letter S a lot (later, the letters A, J, L, and M also crept in). The results were published in a limited-edition book, Notes Towards The Complete Works of Shakespeare. A researcher reported:
"They were quite interested in the screen, and they saw that when they typed a letter, something happened. There was a level of intention there."
This theorem has been traced to the mathematician E'mile Borel in 1913, and was first popularized by the astronomer Sir Arthur Eddington. It became part of the idiom of techies via the classic SF short story "Inflexible Logic" by Russell Maloney, and many younger hackers know it through a reference in Douglas Adams's "Hitchhiker's Guide to the Galaxy".
On 1 April 2000 the usage acquired its own Internet standard, RFC2795 - Infinite Monkey Protocol Suite.