I would like someone with more knowledge of themeing to review the posts at http://drupal.org/node/147374

I (coreb) gave some suggestions on how to modify a "Who's Online" snippet to do what they wanted. I know that there's a better way to do it by overriding theme_user or using some sort of hook_*_alter, but I don't understand how. Theme overrides have been something I haven't been able to grok.