I recently switched to zsh on my Terminal.app on my OS X machine successfully. The version number of zsh is 4.3.11.
If you're not using Homebrew, this is what I just did on MAC OS X Lion (10.7.5):
Get the latest version of the ZSH sourcecode
Untar the download into its own directory then install: ./configure && make && make test && sudo make install
This installs the the zsh binary at /usr/local/bin/zsh
.
You can now use the shell by loading up a new terminal and executing the binary directly, but you'll want to make it your default shell...
To make it your default shell you must first edit /etc/shells
and add the new path. Then you can either run chsh -s /usr/local/bin/zsh
or go to System Preferences > Users & Groups > right click your user > Advanced Options... > and then change "Login shell".
Load up a terminal and check you're now in the correct version with echo $ZSH_VERSION
. (I wasn't at first, and it took me a while to figure out I'd configured iTerm to use a specific shell instead of the system default).