We're slated to hit 7,000,000,000 in 2012.
We hit 6 billion in 1999.
Thirteen years to add a billion.
We hit 1 billion in 1800.
In 1930, 130 years later, we made 2 billion.
(data from the AP)
So between 1930 and 1999 we added 4 billion in 69 years.