Some History and Related Links
The idea that the universe may be a cellular automaton did not originate with Wolfram. Juergen Schmidhuber makes the case that Konrad Zuse had the idea that the universe is running on a grid of computers as early as 1967. Schmidhuber discusses this on his website: Zuse's Thesis: The Universe is a Computer. On the other hand, Plamen Petrov, being unaware of Zuse's work at the time, earlier dubbed this idea Fredkin's Thesis. Recently Schmidhuber observed, "Even earlier, Gottfried Wilhelm von Leibniz (who not only co-invented calculus but also built the first mechanical multiplier in 1670) caused a stir by claiming that everything is computable". Although it seems clear that Leibniz did not formulate the notion of a cellular automaton.
On the other hand, apparently Wolfram does not intend to imply that the universe is a classical cellular automaton--at least if you read the fine print. In the Notes for Chapter 9 on pages 1026 and 1027 of his book Wolfram does acknowledge the work of Zuse and Fredkin in a single sentence. Then he goes on to say that "no literal mechanistic model can ever in the end realistically be expected to work." I take this to refer to classical cellular automata. In his usual modest way he says, "...what must happen relies on phenomena discovered in this book--and involves the emergence of complex properties..."
In addition to the links above those new to this area may wish to read the April 1988 Atlantic Monthly article Did The Universe Just Happen by Robert Wright.
Donald Greenspan is also a long time active researcher in the area of discrete models of physics whose work is unacknowledged by Wolfram. See the Introduction to Greenspan's book Discrete Models which reads like a manifesto for digital physics.
More links of interest: