Here's the bash incantation for that:

    command_exists __git_ps1 && git="$c_vcs\$(__git_ps1)"
    PS1="\n$c_path\w$git $c_id\u@$prompt_host$c_reset"

$c_* vars are just for colour, the command_exists switch lets me use the same dotfile on boxes which don't have git installed.