smäll has an alphabet poster ordered according to popularity for each letter in Google search. (Their site is in Spanish, try using Google translator to read it in English). Without thinking much, you would probably guess that the one letter words A and I are miles ahead of any other searched letter (over 25 billion each). It falls off very quickly, with S, T and E at 4 billion (smäll speculates that S is third as it’s the abbreviation of saint and street). Look here to see the number of searches for all letters and the numbers 0-9.
Maybe more interesting would be an alphabet poster of the most searched words starting with each letter of the alphabet. (Check out Google Zeitgeist to see some of the phrases that were used most in searches in 2010.)