> For that matter, what is your character set? Are you using just letters, or alphanumeric characters? Again, I'd prefer the former since you aren't going to get a significant boost from the extra choices. You might also want to take out 'O', 'o', and 'l' so its unambiguous when written out.
I'd suggest using New Base 60, which has some smart readability built in: http://tantek.pbworks.com/w/page/19402946/NewBase60