I keep running into people talking about the "20% chance we're living in The Matrix thing." The New York Times Science section published something on the Oxford University philosopher touting the theory that it's plausible we're just a function of someone else's simulations:

"This kind of posthuman might have other ways of having fun, like stimulating their pleasure centers directly," Dr. Bostrom says. "Maybe they wouldn't need to do simulations for scientific reasons because they'd have better methodologies for understanding their past. It's quite possible they would have moral prohibitions against simulating people, although the fact that something is immoral doesn't mean it won't happen."

Dr. Bostrom doesn't pretend to know which of these hypotheses is more likely, but he thinks none of them can be ruled out. "My gut feeling, and it's nothing more than that," he says, "is that there's a 20 percent chance we're living in a computer simulation."

You can check out Dr. Bostrom's site here. And here's the abstract from his paper, "Are You Living in a Computer Simulation?"

This paper argues that at least one of the following propositions is true: (1) the human species is very likely to go extinct before reaching a posthuman stage; (2) any posthuman civilization is extremely likely to run a significant number of simulations of their evolutionary history (or variations thereof); (3) we are almost certainly living in a computer simulation. It follows that the belief that there is a significant chance that we will one day become posthumans who run ancestor-simulations is false, unless we are currently living in a simulation. A number of other consequences of this result are also discussed.

And feel free to add to that number...