It looks like add() function is truly 100% atomic, and safeadd bicycle mentioned in the other comment is useless. There are few links where developers of Memcahed explained it deeper

http://lists.danga.com/pipermail/memcached/2008-March/006647.html
http://www.serverphorums.com/read.php?9,214222

Race conditions happen on an heavy load server when more than one thread tries to execute memcache_add.
For example if thread A and thread B try to save the same key you can test that sometimes both return TRUE.
To have the right behaviour you can verify that the correct value is in the assigned key:

<?php
function memcache_safeadd(&$memcache_obj, $key, $value, $flag, $expire)
{
    if (memcache_add($memcache_obj, $key, $value, $flag, $expire))
    {
        return ($value == memcache_get($memcache_obj, $key));
    }
    return FALSE;
}
?>

skeleton of a thread safe updater for an incremental counter:

<?php

$key = "counter";
$value = $memcache->increment($key, 1);
if ($value === false) {
   // --- read from DB ---
   $query = "SELECT value FROM database";
   $result = mysql_query($query);
   $row = mysql_fetch_assoc($result);
   $db_value = $row["value"];
   $add_value = $memcache->add($key, $db_value + 1, 0, 0);
   if ($add_value === false) { 
      $value = $memcache->increment($key, 1)
      if ($value === false) {
          error_log ("counter update failed.");
      }
   } else { 
      $value = $db_value + 1;
   }
}

// --- display counter value ---
echo $value;

?>

It's also good to note that add will succeed if the key exists but is expired

[c.2007]
if you read source code for MMC_SERIALIZED you will see at line ~1555 that [a line ~1560]
!(is_string,is_long,is_double,is_bool)

[is] serialized and that serialized values are flaged as MMC_SERIALIZED for return (fetch) code unserialize these values again