Root's shell is /sbin/sh, which is statically linked.
Don't just insert a 'c' before "sh" as previously, as that would
look for /sbin/csh, which doesn't exist. Don't just change it to
/bin/csh, since that's really /usr/bin/csh, which is dynamically
linked, because:
1) /usr may not be mounted initially, and then
you're in deep (the shared libraries are in /usr!), and
2) There is code in the startup scripts that assumes that
everything critical is in /etc/lib, not /usr/lib.
Approach with caution!
Whenever you change root's shell, make sure you do it using vipw or "passwd -e". Both programs will check for a valid (in the sense of /etc/shells) file.
Safer bet - have an alternate root account, like "rootcsh", with uid 0, and /bin/csh as its shell. Put it after root's entry in the passwd file. Only drawback: you now have to remember to change all of root's passwords at the same time.
Third bet - in root's .profile, check if /usr is mounted and, if so, exec /bin/ksh or whatever.
An even better bet - leave root's shell alone, and use the sudo command for doing things as root.