The U.S. Census estimated that the population of the United States reached 300 million today. From the census page, the statistics for October, 2006 estimate:
One birth every 7 seconds
One death every 13 seconds
One international migrant (net) every 31 seconds
Net gain of one person every 11 seconds
It took 39 years to grow from 200 million in 1968 to the current population. To grow from 100 to 200 million took 54 years from 1915.
The current growth rate for the U.S. population is roughly 1% per year.
The current world population is estimated at 6.551 billion, and estimated to reach 7 billion around 2012.