What I'd like to know is why some letters have higher probability than others? Is that because some letters are more likely on the elliptic curve vs not? If you plug sequences into vanitygen it will give different difficulties depending on letter, eg.

11... > 1A...
1Q... > 1D...
1R... > 1Q... and so on.

I made a table of the first three chars after 1 to help me in a project with fast lookup of difficulty calculation. There are broad classes of probabilities depending on what letters are involved. I guess there is some non 1-1 mapping from 2^160 possible addresses and base 58 representations.