I changed root's shell, forgotten root's password, and I can't login.

If root no longer has a valid shell, your only recourse is to
boot single user from CD.

You need to mount the root file system and fix <mntpoint>/etc/passwd.

If you run NIS or NIS+, you have an easier solution; just add an
account with uid 0 but with a different username and a valid shell.
Use that account to fix the problem with the root account.