Backend Login window can now be templated by extensions. You can find an easy how to here: http://danosipov.com/blog/?p=100
More info:
RFC #8534: Make Backend Login Templateable
http://bugs.typo3.org/view.php?id=8534
RFC #11356: Backend login news is not templateable
http://bugs.typo3.org/view.php?id=11356