Index: wp-includes/pluggable.php
===================================================================
--- wp-includes/pluggable.php	(revision 12716)
+++ wp-includes/pluggable.php	(working copy)
@@ -1575,7 +1575,10 @@
 		if ( $user )
 			$email = $user->user_email;
 	} elseif ( is_object($id_or_email) ) {
-		if ( isset($id_or_email->comment_type) && '' != $id_or_email->comment_type && 'comment' != $id_or_email->comment_type )
+        if ( isset($id_or_email->comment_type)
+           && '' != $id_or_email->comment_type
+           && ! in_array( $id_or_email->comment_type,
+              apply_filters( 'get_avatar_comment_types', array( 'comment' ), $id_or_email->comment_type ) ) )
 			return false; // No avatar for pingbacks or trackbacks
 
 		if ( !empty($id_or_email->user_id) ) {