Leopard uses dscl, directory services command line, tool to manage most things important. Here is how you switch to Z-Shell:
sudo dscl /Local/Default -create /Users/ngift UserShell /bin/zsh
'chsh -s /bin/zsh' still works too.
I originally used chsh to change the shell for my terminal, but it didn't also change it for X11 strangely. I suppose dscl is the universal bridge to modify any local directory attribute now, so in the long run it is probably the best tool to use for everything from adding mount records to modifying managed preferences, either locally or connected to a directory system.
There is even a GUI for this (!): Ctrl-Click your icon in the user preferences pane and choose "Advanced Options
what's wrong with chsh(1)?
nvm, should've read the comments first.
1 - Why switch?
2 - Does it cause any problems?
Robert/I wrote an article here about it here .
Basically, Z-Shell has very nice auto-completion stuff built in. I find myself doing many less keystrokes per day. Z-Shell is also even more configurable than bash, while retaining all of the good features of bash.
It doesn't cause any problems to change your user shell, as this won't affect the system shell.
"I originally used chsh to change the shell for my terminal, but it didn't also change it for X11 strangely"
Sounds like you might have a more serious issue. I used chsh(1) in Terminal.app and the settings were effective in X11. A check of Advanced Options in User Preferences also showed the change as well.
My advice: 1) Do a clean install, or, 2) Do an archive and install.