Yoast.com actually covered this quite recently…
if ( is_singular() ) {
global $post;
if ( current_theme_supports('post-thumbnails')
&& has_post_thumbnail( $post->ID ) ) {
$thumbnail = wp_get_attachment_image_src(
get_post_thumbnail_id( $post->ID ), 'thumbnail', false);
echo '<meta property="og:image"
content="'.$thumbnail[0].'" />';
}
echo '<meta property="og:title"
content="'.get_the_title().'" />';
if ( get_the_excerpt() != '' )
echo '<meta property="og:description"
content="'.strip_tags( get_the_excerpt() ).'" />';
}
Explanation and more details are here: http://yoast.com/facebook-share-wordpress/