/**
 * return post thumbnail inside style in head
 */
if ( !function_exists('fb_background_image') ) {

	function fb_background_image() {
		// only frontend
		if ( is_feed() || is_trackback() ) {
			return;
		}

		// default image, when no set an post thumbnail
		if ( !has_post_thumbnail($GLOBALS['post']->ID) ) {
			$style = '<style type="text/css">.banner {background-image:url(\'' . get_bloginfo('stylesheet_directory') . '/images/banner.jpg\');} </style>' . "\n\n";
		} else {
			// get post thumbnail
			$image = wp_get_attachment_image_src(
				get_post_thumbnail_id($GLOBALS['post']->ID),
				'banner' // size for image; defined via add_image_siz
			);

			$style = sprintf(
				'<style type="text/css">.banner {background-image:url(\'%s\');}</style>',
				esc_attr($image[0]),
				"\n\n"
			);
		}
		if ($style)
			echo $style;
	}

	// add functions
	add_action( 'wp_head', 'fb_background_image' );
	add_theme_support( 'post-thumbnails' );
	add_image_size( 'banner', 980, 350, true );

}

// Load in support for template functions which the theme supports
require_if_theme_supports( 'post-thumbnails', ABSPATH . WPINC . '/post-image-template.php' );

In functions.php it queries whether the theme supports post-thumbnail function:

if ( function_exists( 'add_theme_support' ) )
	add_theme_support( 'post-thumbnails' );

If you look at the file wp-includes/post-image-template.php, there are 4 new features:

• has_post_image()
• get_post_image_id()
• the_post_image()
• get_the_post_image()

The theme itself can check within the loop, if the post has a Post-Thumbnail, and if so, then it can be output:

<?php if ( has_post_image() ) : ?>
	<a href="<?php the_permalink(); ?>"><?php the_post_image( array( 75, 75 ), array( 'class' => 'alignleft' ) ); ?></a>
<?php endif; ?>

Those interested in more features and information should check the file in WordPress 2.9: wp-includes/post-image-template.php
Especially the function: get_the_post_image
It allows to adjust the output by using hooks.
In get_the_post_image the function wp_get_attachment_image get also used, what has been explained in detail in this post just a few days ago.