it is the same method you've probably read about to add an image. create a function with your new code you want to add.... then place it on a hook w/ the add_action function.
while i'm not sure about autofocus, in thematic- the blog description in just under the blog title by default. you can change this text in your WP Settings.
back to adding stuff...
function bacon(){ ?>
<span>Build a Bacon Fortress to Keep the Haters Away</span>
<?php }
add_action('thematic_header','bacon',6)
the add_action function breaks down like this:
add_action('hook you want to add to','your function',priority number)
where priority number isn't necessary if you are adding to an empty hook, but is responsible for ordering functions that are added to the same hook.
a nice diagram of hooks
http://www.bluemandala.com/thematic/thematic-structure.html
visualizing.thematic4you.com