It's the one Harry Lorayne came up with. It could be any other mapping you choose, as long as you stick with it. However, in this method the use of consonants for numbers and vowels as fill-ins is probably already optimal. The Life Hacker post describes some other methods which I'm not familiar with. I plan to try them out to see if they are any more efficient. Memorizing a deck of cards in just over a minute, as described there, is very compelling. I've always required a little bit more time than that to do the same.