table.cc:

  Try to open an old 5.0 table name
  if encoded name doesn't exist.
  This makes SELECT working without having
  to rename tables with tricky characters
  in name into 5.1 format (simpliefies upgrade).

sql/table.cc

@@ -270,7 +270,32 @@ int open_table_def(THD *thd, TABLE_SHARE *share, uint db_flags)
 
   strxmov(path, share->normalized_path.str, reg_ext, NullS);
   if ((file= my_open(path, O_RDONLY | O_SHARE, MYF(0))) < 0)
-    goto err_not_open;
+  {
+    /* Try unecoded 5.0 name */
+    uint length;
+    strxnmov(path, sizeof(path)-1,
+             mysql_data_home, "/", share->db.str, "/",
+             share->table_name.str, reg_ext, NullS);
+    length= unpack_filename(path, path) - reg_ext_length;
+    /*
+      The following is a safety test and should never fail
+      as the old file name should never be longer than the new one.
+    */
+    DBUG_ASSERT(length <= share->normalized_path.length);
+    /*
+      If the old and the new names have the same length,
+      then table name does not have tricky characters,
+      so no need to check the old file name.
+    */
+    if (length == share->normalized_path.length ||
+        ((file= my_open(path, O_RDONLY | O_SHARE, MYF(0))) < 0))
+      goto err_not_open;
+
+    /* Unencoded 5.0 table name found */
+    path[length]= '\0'; // Remove .frm extension
+    strmov(share->normalized_path.str, path);
+    share->normalized_path.length= length;
+  }
 
   error= 4;
   if (my_read(file,(byte*) head, 64, MYF(MY_NABP)))