$selected_content_types = array('page' => 'page', 'story' => 'story');

This query returns results:

$result = db_query("SELECT n.nid, n.type, n.title FROM {node} n WHERE  n.type IN ('page','story')";

But, neither of these queries do:

$result = db_query("SELECT n.nid, n.type, n.title FROM {node} n WHERE  n.type IN (%s)", implode(',', $selected_content_types));

$result = db_query("SELECT n.nid, n.type, n.title FROM {node} n WHERE  n.type IN (%s)", "'" . implode("'","'", $selected_content_types) . "'");