Well, you'll have to make a custom header.php in you child-theme-directory which does not display the header-information. You could make this into:
<?php thematic_create_doctype(); echo " "; language_attributes(); echo ">\n";?>
<head profile="http://gmpg.org/xfn/11">
<?php
thematic_doctitle();
thematic_create_contenttype();
thematic_show_description();
thematic_show_robots();
thematic_canonical_url();
thematic_create_stylesheet();
thematic_show_rss();
thematic_show_commentsrss();
thematic_show_pingback();
thematic_show_commentreply();
wp_head(); ?>
</head>
<body class="<?php thematic_body_class() ?>">
<?php thematic_before(); ?>
<div id="wrapper" class="hfeed">
<div id="main">
Then you'll have to add that information to your sidebar. To do this add the following to your functions.php:
<?php
function add_sidebar_header() { ?>
<?php thematic_aboveheader(); ?>
<div id="header">
<?php thematic_header() ?>
</div><!-- #header-->
<?php thematic_belowheader(); ?>
<?php } ?>
<?php add_action ( 'thematic_abovemainasides', 'add_sidebar_header' ); ?>
If all goes well, then your header should be within your sidebar. There could be an error in the code since I'm just writing it as it comes up, haven't checked it. The idea behind my solution is to remove the header above the content and to add it with a function to the hook thematic_abovemainasides.