I have no idea how I came across this the other day but I found it mildly interesting. And by that I mean it leaves you thoughtful for a few fleeing moments, but then you basically forget about it and carry on with your life as normal. Anyway, the following is taken mostly from the wikipedia article outlining philosopher Nick Bostrom's idea:

The Premise

Bostrom's argument first requires us to make the following assumptions:

1. It is possible that some day, a civilization (us, or anybody in the universe really) would have the technical ability and resources to create a computer-based simulation containing self-aware intelligent entities.
2. It is likely that this civilization would run a huge (~10⁹ and up etc.) amount of the simulations, for observing behavior, research, or whatever.
   
3. The nature of the simulation is such that an individual inside the simulation would be unable to detect that they are not in "the real world", as their entire reality as perceived by them (IE a false reality) is provided solely by the simulation.

Basically, think The Matrix. From here, if one assumes that 1-3 are at least remotely possible, then which of the following is more likely?

1. We are the one real civilization that will eventually develop the simulation, or
   
2. That we are one of the billions of simulations that have run?

If you agree 1-3 are at least possible, the the following trichotomy emerges:

1. Intelligent species will never reach a level of technology to create simulations accurate enough to be mistaken for reality, or
2. Species who reach this level of technology and are capable of running such simulations, don't tend to do so, or
3. We are almost certainly living in a simulation.

Uh... Okay?

So essentially, this entire thing hangs on assumptions 1-3. Assuming humanity doesn't wipe itself out, maybe those aren't that huge of assumptions. Interestingly enough, even if you consider the probability that 1-3 are true to be very small, the vast numbers of simulations that would be run still make part 3 of the trichotomy very very likely.

Like I said, think about it, let it marinate, and then go on with your life. This sort of thing is just one of many skeptical hypotheses, a theory that basically says we have no idea what the hell is really going on with life.

Interesting...