In watching the Discovery Channel’s show on Rutan’s X-Prize effort, they mention that the White Knight plane is Rutan’s 318th design. It’s tail number is N318KF. Presumably the 318 is for it being his 318th design. Can Rutan go to the FAA and say, “Here’s the tail number I’d like for my plane.” and get it? If so, I have to wonder why SpaceShip One’s number is N328KF, and Rutan has what looks like a F-20 Tigershark in his hanger with tail number N528KF on it.
You can get “special tail numbers” if you submit the request to the FAA.
http://members.eaa.org/home/homebuilders/faq/Registering%20Your%20Homebuilt.html
Nitpick: White Knight is actually N318SL (cite: registry.faa.gov, where you can also reserve a custom N-number for $10), and is indeed Scaled Composites Model 318. Several other aircraft designed and owned by Scaled Composites also had N-numbers assigned based on their model number (133SC, 191SC, etc). Presumably, White Knight would have been 318SC, had that number not already been assigned.
SpaceShipOne’s tail number was a little more symbolic: 328KF = 328K feet = 100km, or the edge of space as far as NASA, the FAA, and the X Prize Foundation are concerned. 
Yep, requests for particular tail numbers is fairly common. In fact, major airlines have a bunch of 'em reserved for their use (United, for example, favors tail numbers ending in “UA”.) It’s the airplane equivalent of a vanity license plate.
You can also change an airplane’s tail number - one of the the airplanes I rent from time to time started life with one tail number and, when bought by the current owner, was changed to a number that ended in two letters representing the company-owner (Rather like United’s “UA” and Rutan’s “SC”). All documented in the paperwork for the airplane.
In both cases, all you need is money and a bunch of properly filled our forms. And not that much money or paperwork.
You can also assign an N-number you own to another plane. A friend of mine did this on request (via Cessna) from a famous golfer. It seems he’d ordered a new Cessna Citation jet and wanted it to have a number that my friend had on his glider. They agreed on a price, and the number was trasferred to the Citation.
I heard that happened with N1KE.
From watching “The Apprentice”, The Donald’s Sikorsky S-76b helo has a tail number of “N76DT”. I think his private jet has a tail number ending in “DT” as well, but there were too many matching records in the AOPA database, and I don’t remember the full number.
It might be registered as N318SL, but the clips I saw last night had N318KF painted on the tail of the plane.
I’m afraid your memory is playing you false, probably because of the “KF” on SS1. This is from Scaled Composites themselves.
Could be, but a lot of the video from the show last night was edited kind of strangely, with shots that had obviously been taken early in the effort (the R&D period) mixed in with shots of the flight, and there was nothing in the narration to indicate that they were doing this. They had rather bizarre shots taken from inside WK, but the way the narration was going, it looked like the shots were from inside SS1, and I kept trying to figure out why the hell there were two helmets in the shots, when there was only supposed to be the pilot inside SS1. So it might be possible (though I don’t know how likely it is), that Rutan applied for the N318KF number, and had painted it on the plane, only to be told later on by the FAA that that wasn’t available, and then switching to N318SL for the final numbers.
It did. N1KE used to belong to Peter Lert, a writer who hangs out in the Usenet rec.aviation.* newsgroups. It was the tail number of his glider, which is now N3ZV.
He posted about it here.
The Udvar-Hazy branch of the Smithsonian Air & Space Museum has the first plane delivered to and used by Federal Express, a Dassault Falcon 20C. The registration number is N8FE. Fred Smith, the founder of Federal Express, wanted any potential competitors to think that he had at least eight aircraft instead of just one.
I looked up that number at the link Shagnasty gave, and there’s the registered owner, NATIONAL AIR AND SPACE MUSEUM
BTW, You can look up N-numbers using Google. Handy.