The question about integrating CAPTCHA into the login is coming up more frequently. With the wpmem_login_form_rows filter, the login form is infinitely flexible so you can add additional elements to the form.
I have put together an example of integrating the Really Simple CAPTCHA process into the current login process. The process is relatively simple, since the plugin contains support for Really Simple CAPTCHA already in the registration form. This customization will utilize some of the already available process in native to the plugin.