This bug didn't exist in 1.1.0dp5, but I've seen it before in php-memcached. The issue is when issuing a setMulti() array into CB it doesn't like it if the key is not a string.
$key = 12345
$value = 67890
$value = json_encode($value);
$array[$key] = $value;
PHP Fatal error: Uncaught exception 'CouchbaseIllegalKeyException' with message 'Invalid key specified (not a string)'
test2.php(67): Couchbase->setMulti(Array, 604800)
The reference bug for php-memcached is: https://bugs.php.net/bug.php?id=59016
After experiencing the issue there I recompiled their newest version and it fixed the problem. I'm not sure if you can dovetail off of that work or not.