I seem to recall that the Navy claimed the letter N and something about the use of W for radio. I'll research and post it if someone else doesn't do it first.

Found this at http://www.skygod.com/asstd/abc.html. With a name like that it naturally has to be the website of the Airline Pilots Association.

"Some special interest groups successfully lobbied the government to obtain their own special letters. The Navy saved all the new 'N' codes. Naval aviators learn to fly at NPA in Pensacola, Florida and then dream of going to "Top Gun" in Miramar, California (NKX). The Federal Communications Committee set aside the 'W' and 'K' codes for radio stations east and west of the Mississippi respectively. 'Q' was designated for international telecommunications. 'Z' was reserved for special uses. The Canadians made off with all the remaining 'Y codes which helps explain YUL for Montreal, YYC for Calgary, etc. (The start of the the song YYZ by the band Rush is the Morse code for the letters Y Y Z. Rush is from Toronto.) One of the special uses for 'Z' is identifying locations in cyberspace. What am I talking about? Well, an example is ZCX the computer address of the FAA's air traffic control headquarters central flow control facility. ZCX is not an airport but a command center just outside Washington D.C., that controls the airline traffic into major terminals."