Coming from a jQuery point of view (love this framework!), this is the concept behind the functionality you want:
Create the ‘normal’ EE login form. Using the id of the form, use the jQuery Form Plugin (http://www.malsup.com/jquery/form/) to “ajaxify” the form.
With this plugin you can define what element in the DOM should be updated with the form’s submission result (which will be some kind of template) and voila, done.
E.g.
Assuming that the form as the id ‘login’ (I haven’t used the exact value eludes me) and the div you want the message to appear has id=“login-div”, the code for changing the standard form to Ajax would be:
var options = {target : "#login-div"};
$(document.ready(function()
{
$("#login").ajaxForm(options);
}
(don’t forget that you have to load jquery and the plugin in your html head section before the code, though).
This should (theoretical! I haven’t tried it out) make a the login form Ajax-style without hacking too much 
Check the website of the jQuery form Plugin for more options and details. If you have any problems you can always contact me. If we have created a final, workig version of this, we’ll put it into the knowledge base.
