Great to know that I am not the only letter counter in the world! I started doing this when I was 6 and at that time I simply counted all the letters in any sentence I heard using my fingers, one by one. After a while, I guess I got bored and I stopped doing it altogether. I am 25 now and a couple of years ago I realized that I can count the letters in any word in just a couple of seconds. My surprise was that nobody else was able to do it! I count letters in quite a simple way: when I hear a word (e.g. Popocatepetl), I break it up in blocks of roughly the same sizes (popo + cate + petl) and quickly add them (4+4+4=12). I can do it both in my native language (Bosnian) and in English if I know the spelling of the word. I turn this ability on only if I need it and it does not get in the way of my every day life. I also have a pretty high IQ and I read extremely quickly, but I am not sure if these things are related or not.