Aha - I knew this sounded familiar. For those not familiar with it, here it is:

Although you might see people from time to time, they are most likely products of your imagination. Simple mathematics tells us that the population of the Universe must be zero. Why? Well given that the volume of the universe is infinite there must be an infinite number of worlds. But not all of them are populated; therefore only a finite number are. Any finite number divided by infinity is zero, therefore the average population of the Universe is zero, and so the total population must be zero.

Retrieved from http://hitchhikers.wikia.com/wiki/Universe

EDIT: note that this doesn't work, for several obvious reasons, notably that a subset of an infinite set can be infinite.