It might be nice to do it client-side.

    var caps = null;
    passwordField.onkeypress = function(e) {
        if (caps !== false) {
            var s = String.fromCharCode(e.keyCode || e.which);
            caps = s.toUpperCase() == s && !e.shiftKey;
        }
    };
    loginForm.onsubmit = function(e) {
        if (caps) {
            passwordField.value = invertCase(passwordField.value);
        }
    };