This looks like an obvious mistake.
$fields probably should be $name in this case.

diff -Nurp ../cck.orig/modules/fieldgroup/fieldgroup.module ./modules/fieldgroup/fieldgroup.module
--- ../cck.orig/modules/fieldgroup/fieldgroup.module	2010-06-25 15:24:34.000000000 -0500
+++ ./modules/fieldgroup/fieldgroup.module	2010-06-25 15:25:31.000000000 -0500
@@ -729,7 +729,7 @@ function fieldgroup_nodeapi(&$node, $op,
         $fields = $node->content;
         if (!empty($fields)) {
           foreach ($fields as $name => $more) {
-            if (is_string($fields) && strstr($fields, 'field_')) {
+            if (is_string($name) && strstr($name, 'field_')) {
               $field_rows[] = $name;
             }
           }