Try this:

  export PS1='\w:$(__git_ps1 "%s")$ '

EDIT - here's mine btw:

  function parse_git_dirty {
    git diff --quiet HEAD &>/dev/null
    [[ $? == 1 ]] && echo "✎ " || echo "➜ "
  }
  function parse_git_branch {
    local branch=$(__git_ps1 "%s")
    [[ $branch ]] && echo "$branch$(parse_git_dirty)"
  }
  PS1='\u@\h $(parse_git_branch)\W $ '

If the branch has local changes it shows the pencil, in addition to the branch name. I'm still experimenting.