How many people 

How many people 

Earth current pop
10^{10} x 0.577  ±<1% ??? 
CIA July 1996 estimate [link broken  new site] 
10^{10}  +73%  
6 x 10^{9}, six billion  +4% 
US current pop
10^{8} x 2.66  ±<1% ??? 
CIA July 1996 estimate [link broken  new site] 
10^{8}  62%  
3 x 10^{8}, three hundred million  +13%  
23 x 10^{8}, two to three hundred million  25% to +13%, a 38% spread  
2.5 x 10^{8}, two hundred fifty million  6%  
2.7 x 10^{8}  +2% 
City pop
1 to 30 million 
10^{7} big, 10^{6} mid, 10^{5} small a million ± x 10 
Earth cumulative pop
10^{11}  ×0.22 
Say 1 life / 25 or 50 years, or 24 lives / 100 yr.
1900 to present, 24 lives, 2 growing to 6 billion pop, say 10 billion. (424)
1800 to 1900, 24 lives, 1 to 2 billion, say 4 billion. (28)
1000 to 1800, 1632, 3 to 8 10^{8}, say 15 billion. (526)
1 to 1000, 2040 lives, 3 × 10^{8}, say 10 billion. (612)
10kYA to 2kYA, 160320 lives, 10^{7 to 8}, say 10 billion. (232)
50kYA to 10kYA, 8001600, 10^{4? 6?}, say <~1 billion. (01)
600kYA to 50kYA, 12 × 10^{4} gens, 10^{4? 5?}, say <~1 billion. (01)
Adding it up... say 50 billion (19104). Say 0.5 × 10^{11} ±×2.
I'm going to call it 10^{11}.
The low bound unfortunately falls out of 10^{11} into 10^{10.}.
Reducing the `how old before counted' threshold would nudge it back upward.
I am looking around for a closer estimate.
I note that roughly (<x2) about as many people lived >2kYA, AD
11000, 10001800, 1800present.
Hmm... I wonder how much of our increasing rate of technological progress is simply the result of more warm bodies...?
My very first cut was this... 
If 1 generation per 20 yr, homo sap: 10^{6} x 10^{say 5} / 20 ~= 10^{~11} modern: 10^{5} x 10^{say 6} / 20 ~= 10^{~11}
Better homo sap: 10^{5 to 6} (yrs) x 10^{4 to 8} (ave
pop) / 10 ~= 10^{8 to 13}.

Further notes (1998.May.11):
"The dead outnumber the living, in a ratio that could be as high as 20 to 1," a demographer, Nathan Keyfitz, wrote in a 1991 letter to the historian Justin Kaplan. "Credible estimates of the number of people who have ever lived on the earth run from 70 billion to over 100 billion." 
The Wreck of Time  Taking our century's measure
by Annie Dillard Harper's Magazine, January 1998, p 51 
How Many People Have Ever Lived On Earth?, and a related note[broken].
How many people have ever lived?
Estimating The Number of People That Have Ever Lived
Historical Estimates of World Population
Historical World Population Estimates
Comments encouraged.  Mitchell N Charity <mcharity@lcs.mit.edu> 
Notes: Doables: link to relevant current factbook pages, rather than to site. area density  crowd estimation  4/m^2 shoulder to shoulder "100%", 6/m^2 pressing "150%", sitting "25%", walking freely "10%". Sketch area, assign densities, calc crowd. History: 2003Dec05 Repaired links  1 flagged, 1 fixed, 4 added. 2003Feb04 Repaired links  2 flagged, 1 fixed, 1 removed, 2 added. 2001Feb11 Fixed same link. 1999.Feb.15 Added link to PRB's `How Many People Have Ever Lived On Earth?'. 1998.Mar.11 Added Keyfitz's cumulative human pop estimate. long ago Created.