Using inotifywait.

watch-todo.sh

watch_todo(){
        tput smcup
        while : ; do
                tput clear
                echo -n "${bg[cyan]}[[[ TODO: ]]]$reset_color\n"
                if [[ -n $1 ]]; then
                        todo.sh "$@"
                else
                        todo.sh ls
                fi
                trap 'break' 2
                inotifywait -e modify $HOME/Dropbox/t/todo.txt -qq
        done
        tput rmcup
}