On Ubuntu 4.4.0-34, the shell in /etc/passwd only applies  to  logins
from  outside,  but  doesn't  affect  Terminal  windows.  To get your
preferred shell in a new Terminal window, you must use  the  menu  at
the top of the screen.  Go to
   Edit -> Profile Preferences
This pops up the "Editing Profile "<name>" window. Select the Command
tag.  Check the
   Run a custom command instead of my shell
item, and type the path to your shell in the "Custom Command:"  entry
widget.

This is all somewhat counter-intuitive, but it seems to be  the  only
(documented) way to get shell other than bash in a Terminal window.