Hook midway through the_content();?

Use this function ad_content() in replace of the_content() in your template.

function ad_content() {
$content = apply_filters ( 'the_content', get_the_content () );
$content = explode ( "</p>", $content );
$half_way = ( count($content) / 2);
    for ( $i = 0; $i < count( $content ); $i ++ ) {
        if ( $i == $half_way ) {
            echo 'YOUR_AD_CODE';

        }
        echo $content[$i] . "</p>";
    }
}