Not real sure how this bandwagon got started and I don't have time to read all the posts, but having a single user is by no means secure! Why do you think that nix has a root account and windows an admin account? No one in their right mind would run a linux box in single user mode and let the servers be run on the root account. All a cracker would have to do is get a shell through an exploit or a buffer overflow and then they have root access. All you have done is save them the extra step of trying to get root access. The whole point of a multiuser system is that the system owner can reduce the abilities of other users and processes that don't need certain abilites.

I'm riding with chsh on this one; time to think of a better idea guys!