hide Posted by, but show date

You can override the theming functions: http://api.drupal.org/api/function/theme_node_submitted/6 and http://api.drupal.org/api/function/theme_comment_submitted/6

This is really easy, actually. Assuming you're using a phptemplate-based theme you can print either the creation date or creator's username alone using the variables $date and $name respectively.

For example just put this somewhere in your node.tpl.php file:
print $date;