I read on the CNN website  that the Census Bureau  expects the population of the United States to cross the 300 million mark at approximately 7:46am ET tomorrow. Wow! We hit 200 million in 1967, and we hit 100 million in 1915. The Census Bureau has an interesting Facts for Features  detailing statistical changes over the years.
Most interesting numbers (IMHO): Cost for a gallon of regular gas
2006: $3.04 (as of Aug. 7)
1967: 33 cents ($2.00 in 2006 dollars)
1915: 25 cents ($5.01 in 2006 dollars)
Watch the changes live with the US POPClock .