I found Will's entry below so fascinating, I felt the need to return to colonial times and add on the following interesting fact:
Today is also the day, back in 1790, that the very first official US census got underway. Why it took nearly 15 years from the official signing of the Declaration of Independence to the official tallying of the people in the Union is mysterious, but perhaps their computers were a little slower back then.
Anyway, here's what that census yielded:
Estimated population of the United States = ca. 4 million (compared to ca. 300 million today)
Number of people per square mile = ca. 4.5 (compared to ca. 80 today)
Third biggest city = Boston, with ca. 18,000 people already mispronouncing words like "car" and "yard."
Second biggest city = New York City, with ca. 33,000 people desperately trying to hail what few cabs there must have been at rush hour.
Number one biggest city in the Union = Philadelphia, of course, with ca. 42,000 inhabitants, no doubt all of them frustrated beyond belief with their city's sports teams.