There is at least one extension that allows you to modify the back end url..
The one I use allows you to set it up so that the admin url is
www.mysite.com/administrator/index.php?yoursecretword instead of www.mysite.com/administrator
If you don't put the parameter in it will simply return you to the homepage
That has an added benefit in that the script kiddies will usually not keep attacking and sucking your resources if they can't get to the login page.