Editing the Log In form
Using Jamroom
Note that I've also added From Designer support to the /user/login view for the next release. This would allow you to create a new custom "notice" form type and put in a custom message or HTML.
Still needs some testing on this end, and adding custom FIELDS to the login form is not going to get you anywhere, but it might helpful to those using custom modules.
For a FULLY custom login you need to do like Douglas outlined.