Very interesting article:
http://www.prb.org/Articles/2002/HowManyPeopleHaveEverLivedonEarth.aspx
As of 2002, here were the best guesstimates:
Number who have ever been born 106.5B
World population in mid-2002 6.215B
Percent of those ever born who are living in 2002 5.8%
I_Am_Xenon
Pro 
Hrm. Interesting.
So Philip Jose Farmer got it wrong in his Riverworld books, then. He had the Riverworld populated with everyone who had ever lived, provided they died before the early 1980's. Word had it that an identical world was populated with those who died after the first Riverworld's cutoff, and that the populations were about equal.
Then again it wasn't exactly made clear how far forward this second, hypothetical Riverworld would go. Or if it did, I have long since forgotten as it has been several years since I last read the series.