According to a recent article in Nation's Restaurant News, 44% of all American restaurants offer hamburgers on the menu. 14% of all restaurant meals are hamburgers, and 42% of those orders are for bacon cheeseburgers.
A good tour of the best burgers in the U.S. can be found in the book Hamburger America or on the author's blog about his travels tasting burgers all over America.
Despite a growing number of American restaurants in Prague, I haven't found many really great burgers here. Brewsta of the Czech Please blog wrote an article on the burgers of Prague in February 2008. I may take a tour of these places (and a few more) and see for myself where the best burger in Prague can be found.