Use the old drupal_set_title('') function to set the title and put it in hook_form_alter. Why in a hook_form_alter? If you do it in preprocess page your 're to late, if you want it to do with drupal_set_title(). But what you can do in the preprocess page is set $vars['title'] (thanks to Davy aka the drupalcoder to point this out). Using hook form_alter has the advantage that it's independent of the theme.
