Where did the US 'N' registration letter come from? Why is the first letter in US registration numbers 'N'? Other countries seem to use a letter significant to their country, for instance 'G' for Great Britain, 'F' for France, 'D' for Germany (presumably for Deutschland), and 'JA' for Japan.

Does 'N' have any significance at all?
The origins of this can be traced back to CINA (the Commission Internationale de Navigation Aerienne / the Convention for the Regulation of Air Navigation) established as part of the Paris Peace Conference immediately after WWI.

A part of this conference was the adoption of a system of international aircraft identification still in use today. The U.S. delegation was allocated "N".

A number of theories exist as to why "N" was chosen specifically, and there's a great article about it over at the American Aviation Historical Society.

Theories range from, the largest number of states begin with the letter N; to the recognition of the development of wireless communications by the U.S. Navy that had been using "N" as the prefix to its station call-sign identifiers since 1909.