Yesterday we releasing Drupal module named Reform that allows us to change some of Drupal core default components. This is to fill in some missing features in Drupal 7 itself, such as ability to limits the minimum username length and maximum username length.
With this module we can also change the label, description, and the button on the registration page, login page and password request page. Far from perfect since the module still on Alpha stage but at least it works. Visit the Reform project page here.