I don’t know of a theme that does that, but it is fairly easy to modify some themes. For example, the TwentyTen theme can be modified by changing header.php and some of the CSS.
First, move the #access div to follow the body tag like this:
<body <?php body_class(); ?>>
<div id="access" role="navigation">
<?php /* Allow screen readers / text browsers to skip the navigation menu and get right to the good stuff */ ?>
<div class="skip-link screen-reader-text"><a href="#content" title="<?php esc_attr_e( 'Skip to content', 'twentyten' ); ?>"><?php _e( 'Skip to content', 'twentyten' ); ?></a></div>
<?php /* Our navigation menu. If one isn't filled out, wp_nav_menu falls back to wp_page_menu. The menu assiged to the primary position is the one used. If none is assigned, the menu with the lowest ID is used. */ ?>
<?php wp_nav_menu( array( 'container_class' => 'menu-header', 'theme_location' => 'primary' ) ); ?>
</div><!-- #access -->
<div id="wrapper" class="hfeed">
Then add this CSS to the end of style.css:
#access {
width: 100%;
position: fixed;
}
#wrapper {
margin-top: 0;
}