It's actually a speculative metaphysical interpretation of the Feynman path integral formulation of quantum mechanics. Some people call this the "many worlds" interpretation. In view of the fact that to do path integration over all the state vectors you'd need an uncountably infinite number of worlds, "many" is probably the understatement of all time.
It's been going around in various guises since the late 1940's. I don't know if it inspired Douglas Adams or not -- and I didn't have him in mind when I posted -- although I enjoyed his stuff.