I just noticed that Theme Hybrid adds rel="canonical" link to post pages, but it doesn't add them to category pages, for example. Why not add it to all pages?
Honestly, I can't think of any specific scenarios of content duplication on category pages --- but that may be only because of my limited imagination.
Wouldn't it be safer to just add the links in all pages?
Just for the sake of example, Joost de Valk's Canonical URL's plugin ( http://yoast.com/wordpress/canonical/ ) adds them in all site pages.