How do I make search only search in post content?

This line in $wp_query might be of interest:

$where = apply_filters_ref_array('posts_where', array( $where, &$this ) );

You could probably use a filter to toss in an additional post_type=”post”. Something like:

function search_where( $where, &$wp_query ) {
  if ( $wp_query->is_search ){
    where .= " AND post_type="post"";
  }
  return $where;
}
add_filter('posts_where', 'search_where', 10, 2);