The default installation of Red Hat 6.1 (and possibly others - this may be 
a multiple-vendor issue) does not assign a shell for many of the non-user 
(daemon) accounts (e.g. nobody, bin). If no shell is specified, the shell 
defaults to /bin/sh.

This setting is correct for the xfs account, which assigns /bin/false as 
the shell.

The solution would be to assign these accounts a shell which will not 
permit login, such as /bin/false.

Additionally, /etc/shells does not contain a shell such as /bin/false 
which would deny login, which produces an error when assigning such a 
shell to an account via chsh. This entry could be included to /etc/shells 
in the default installation.