Found the answer on SS Bits...

http://www.ssbits.com/tutorials/2009/custom-login-form-with-group-based-redirection/

Make sure you read all the comments to get it working in SS 3.x...

Hi All,

If you want to embed a generic login form on a page in your CMS, you can hijack the default form used by Security/login.

I do this quite a lot, to gain access to the basic form you will need to add the following functions to your page controller:

class Page_Controller extends ContentController
{
...

    private static $allowed_actions = array(
        "LoginForm",
        "login",
    );

    /**
     * Use this function to direct a user where you want them to go after login
     * (currently the root of the site)
     */
    public function login()
    {
        return $this->redirect(Director::absoluteBaseURL());
    }

    /**
     * Get the login form to process according to the submitted data
     *
     * @return Form
     */
    public function LoginForm()
    {
        $authenticator = Authenticator::get_default_authenticator();
        if($authenticator) {
            $form = $authenticator::get_login_form($this);
            // Use this if you want to use a custom form template
            $form->setTemplate('HomeLoginForm');
            return $form;
        }
        throw new Exception('Passed invalid authentication method');
    }

    ...

}

You can add this form to your template by using the $LoginForm variable.

It should work off the bat and load the default login form into your template, if there are any issues give us a shout.