Index: wp-includes/capabilities.php
===================================================================
--- wp-includes/capabilities.php	(revision 15261)
+++ wp-includes/capabilities.php	(working copy)
@@ -1112,7 +1112,7 @@
 
 	$author = new WP_User( $post->post_author );
 
-	if ( empty( $author ) )
+	if ( empty( $author->ID ) )
 		return false;
 
 	$args = array_slice( func_get_args(), 2 );
Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 15261)
+++ wp-includes/ms-functions.php	(working copy)
@@ -209,7 +209,7 @@
 
 	$user = new WP_User($user_id);
 
-	if ( empty($user) || !$user->ID )
+	if ( empty( $user->ID ) )
 		return new WP_Error('user_does_not_exist', __('That user does not exist.'));
 
 	if ( !get_user_meta($user_id, 'primary_blog', true) ) {
@@ -253,6 +253,9 @@
 
 	// wp_revoke_user($user_id);
 	$user = new WP_User($user_id);
+	if ( empty( $user->ID ) )
+		return new WP_Error('user_does_not_exist', __('That user does not exist.'));
+
 	$user->remove_all_caps();
 
 	$blogs = get_blogs_of_user($user_id);
@@ -1326,10 +1329,7 @@
 	}
 	$u = new WP_User( $user_id );
 
-	if ( $u->spam == 1 )
-		return true;
-
-	return false;
+	return ( isset( $u->spam ) && $u->spam == 1 );
 }
 
 function update_blog_public( $old_value, $value ) {
Index: wp-admin/ms-edit.php
===================================================================
--- wp-admin/ms-edit.php	(revision 15261)
+++ wp-admin/ms-edit.php	(working copy)
@@ -264,7 +264,7 @@
 			reset( $newroles );
 			foreach ( (array) $newroles as $userid => $role ) {
 				$user = new WP_User( $userid );
-				if ( ! $user )
+				if ( empty( $user->ID ) )
 					continue;
 				$user->for_blog( $id );
 				$user->set_role( $role );