Light box or not, it's not that very hard to do. I'd go about w/ jquery, ajax, and php.
- Create a hidden div on the page anywhere you want, i'd suggest using fix and then some offset of where you want. Write the html form here for login.
- Create button for login.
<a href="login.php" class="login">Login</a>
- Create Event in Jquery
e.preventDefault(); //Stops from following link
//Add ajax form elements here i suggest this plugin http://malsup.com/jquery/form/
Now from that point in step 3 you're going to do some more magic, when the plugin returns from onComplete, validate correctly that the user has logged in then refresh the page, and there you go.
Now for the 10% of your users who should grow up? Well this even never gets bound so they're taken to login.php.