Index: wp-includes/feed-rdf.php
===================================================================
--- wp-includes/feed-rdf.php	(revision 10885)
+++ wp-includes/feed-rdf.php	(working copy)
@@ -25,7 +25,7 @@
 	<description><?php bloginfo_rss('description') ?></description>
 	<dc:date><?php echo mysql2date('Y-m-d\TH:i:s\Z', get_lastpostmodified('GMT'), false); ?></dc:date>
 	<?php the_generator( 'rdf' ); ?>
-	<sy:updatePeriod>hourly</sy:updatePeriod>
+	<sy:updatePeriod><?php echo apply_filters( 'rss_update_period', 'hourly' ); ?></sy:updatePeriod>
 	<sy:updateFrequency>1</sy:updateFrequency>
 	<sy:updateBase>2000-01-01T12:00+00:00</sy:updateBase>
 	<?php do_action('rdf_header'); ?>