How do you run a shell command/script automatically when entering/cd-ing a directory on Snow Leopard?

Add the following to your ~/.bash_profile:

function cd {
    # actually change the directory with all args passed to the function
    builtin cd "$@"
    # if there's a regular file named "todo.txt"...
    if [ -f "todo.txt" ] ; then
        # display its contents
        cat todo.txt
    fi
}

It's possible you already have a similar function for cd — just extend that one to print the contents of todo.txt if it exists.