Comment by mvilim

  | However, I've been toying around with the idea
  | of eventually writing a minimal 'vi' clone that
  | you cannot exit. I'm sure there's pent-up demand
  | just waiting for one!

If a user space program can be truly unexitable without restarting (including the possibility of killing it through another shell, of course), then would you consider it to be a bug in the operating system? Seems a reasonable conclusion to me.