Multipool 51% attacks

[jwray]

I'm kinda worried that multipool.us is big enough (16GH/s) to easily 51% attack every scrypt altcoin except litecoin and dogecoin, based on these numbers: http://www.coinchoose.com/index.php

Is it definitely possible for a pool operator to run a 51% attack even when all the pool members are running standard legit versions of the mining software?  Is there any way mining software could be upgraded to detect when its pool is trying to run a 51% attack and stop participating in the pool?