Start zsh from bashrc. Useful when you can't use chsh or when the same LDAP account is used both on systems with zsh installed and ones without.

.bash_profile

# .bash_profile is executed for login shells,
# .bashrc is executed for interactive non-login shells.
# We want the same behaviour for both, so we source .bashrc from .bash_profile.
# Also, when .bash_profile exists, bash ignores .profile, so we have to source
# it explicitly.

if [ -f "$HOME/.profile" ]; then
    . "$HOME/.profile"
fi

if [ -f "$HOME/.bashrc" ]; then
    . "$HOME/.bashrc"
fi

.bashrc

# .bash_profile is executed for login shells,
# .bashrc is executed for interactive non-login shells.
# We want the same behaviour for both, so we source .bashrc from .bash_profile.

if [ -z "${NOZSH}" ] && [ $TERM = "xterm" -o $TERM = "xterm-256color" -o $TERM = "screen" ] && type zsh &> /dev/null
then
    export SHELL=$(which zsh)
    if [[ -o login ]]
    then
        exec zsh -l
    else
        exec zsh
    fi
fi

Why do you use : if [[ -o login ]]