Is it possible to sort the user accounts in the login dialog?
This is a great question. A hack has not been created for this yet. The only temporary solution that I know of (I once wanted to do the same thing) is to add spaces to the beginning of a username to sort them (a single space will appear first, two spaces will appear second if I remember correctly).

I would definitely love to see someone find the pref file where it organizes them in alphabetical order so that it can be hacked a bit, although I can imagine it wouldn't be the easiest thing to change and that someone would have to write some sort of script for it.