Mac Terminal: changed my shell, now can't start it

"Open ~/Library/Preferences/com.apple.Terminal.plist and set the ""Shell"" key to ""/bin/bash"", then retry opening Terminal. If this doesn't work, X11.app may be able to open a terminal. Otherwise you'll be stuck making a new user with admin privs and reset your shell there.

Make sure /opt/local/bin/bash is executable by running it in Terminal. If not:

chmod +x /opt/local/bin/bash

Also, /opt/local/bin/bash should be a line in /etc/shells."

The problem here is the /etc/shells file. This file contains a list of all shells that are allowed to be used as login shells. If you want to use the MacPorts-installed bash, you should edit this file to include the path to that particular shell.